First Notes
At first, I advise reading the text-boxes of HoH: Societates, p. 30 (Indirect Spells from ArM5) and p. 35 (Projectiles and Rego Magic).
My purpose here is to discuss a few points involving your question. If you want the fast-answer, go to the Conclusion section, at the end.
Built-in parameters
At first, look at our mantra:
"If a spell is resisted (...) it need not be aimed. If it is not resisted, it must be aimed."(ArM5-Core Rules, p. 86)
Let's stick with the words "need" and "must".
- A spell built in a way it would be resisted by Magic Resistance can freely chose to ignore the Aim Roll, since it is not needed.
- A spell built in a way that would NOT be resisted that way requires an Aim Roll.
The Art of Rego
Now, we need to talk about our friend Rego:
"A spell that has a continuing effect remains in effect even if the caster moves out of range. A spell that allows the caster to control the effect only permits that control as long as the caster is within range. However, it does not expire if the caster moves out of range, and he may control it again if he moves back into range." (ArM5-Core Rules, p. 111: "Ranges")
That said, a Rego spell (including Rego Requisites) with R: Touch can give magi the control over the target as long as it is touching it, and the behavior of the Form follows the influence given for the Duration, even if the magus is not touching it anymore. The "influence" can be an order, for example, in the case of ReMe spells, or, in case of Rego + Elemental Form (e.g.), this is the movement itself, the "kinetic/potential energy" (not in these words for Mythic Europe, but you should recognize it this way).
That means a magus conjuring a Rego Terram spell of R: Touch - D: Momentary (let's keep things solid, for now) can't control the thing beyond their own hands, but the thing follows a movement suggested at the moment of conjuration (that means, it triggers an Aim roll) OR the previous suggestion in the spell description ("always hit the target"), provides the target is within the range of the spell.
Remember that a spell (and a spellcaster) can't exert control further than its range, but it DOES make effect for the duration, and that means magical created things are magical for the duration. If you create a magical rock with R: Touch D: Diameter CrTe and then uses aimed ReTe to throw it, it is Aimed and needs to penetrate, provided the magical nature of the Individual target.
As long as the "auto-hit" is a free choice, not a needed one, a spell can be built to both need an Aim Roll AND overcome Magic Resistance. But what is the advantage of it?
Let''s look at this Mu(Re)Te 10 formulaic spell:
" THE CRYSTAL DART
R: Voice, D: Mom, T: Ind Req: Rego
The spell transforms a stone into a 10-inch crystal dart that rises from the ground at your feet, floats into the air, and speeds off like an arrow at a target that is within Voice range. It does +5 damage (because of the sharp edges of the crystal as much as its speed), and always hits its target, although it must penetrate Magic Resistance to have any real effect." (ArM5- Core Rules, p. 154 + Errata on underlined italic)
The need to overcome Magic Resistance comes from the nature of the thing controlled: a crystal made by magic is a magic thing. The Crystal Dart can be "created" (it is, actually, stone transformed into crystal) at any point within range, and this spell is built in a way the target (the thing controlled: the dart) turns into a magical thing and, then, "suggested" by the spell itself to "hit the target".
Since it is in the description of the spell, you can't choose to make an Aim Roll: it is built to guide the Crystal to the target all the way (otherwise, it would trigger an Aim Roll), but you can guide the projectile in an intelligent way for the duration (for a moment, in this case), as long as the final objective is the same: hitting the target defined at the instant of the casting. The healthiest and more consistent interpretation for the "Momentary" duration is less than a turn, but usually more or equivalent to a glimpse.
Guiding something with pure magical interaction "all the way" usually don't use Finesse Rolls (I say "usually" because I don't know any exception, but if it exists, I won't take this responsibility).
However, you should consider the following guideline of the Limit of Arcane Connections
"Hermetic magic cannot affect an unsensed target without an Arcane Connection."(Arm5- Core Rules, p.80)
That means you should both be in range of the effect AND perceive the projectile.
Note: you can take another example on Piercing Shaft of Wood, a Mu(Re)He 10. (ArM5 - Core Rules, p. 137)
A Note on Arcane Connections to your spells
"Oh so I don't have an Arcane Connection to my own spells?" Well, I'm not 100% sure, but the Arcane Connections table on corebook (p. 84) mentions enchanted devices and mundane crafts.
Further, in a supplement that I can't remember, by the same rules, your Laboratory Texts and other opus are arcane connections to you (that's obvious since they're as mundane as ink and parchment can be, but I didn't realized that a magus should be so carefull before showing them to the world), for the appropriate duration.
If there are nothing stating that you have any Arcane Connections to your spells, I'll give you a reference from this forum:
"A hair from my head was once part of me, so it is an Arcane Connection to me because of the Law of Contagion. I was never part of the hair that came from my head, so by that same Law I am not an Arcane Connection to it." (From here)
That said, if the minor is part of the great, but not the opposite, IN THE BEST CASES the effects are A.C. to the caster, but we won't stick with it. Stay with the opposite: you don't have the Arcane Connection to your effect, so, you need to perceive. Maybe I'm going too far with it, but I couldn't find a clear answer by "ctrl + F" the rulebook, clear my mind if you find something in favor or against this.
How do I controll my effect, since I am both in range and perceiving them?
For the final target changes, we need to consider two situations:
- We consider that the description of the spell requires a definition of the final target before or at the casting instant, as well as the stone that will be target to transform into our Crystal. In this case, we can't change the final target, but can use the moment to guide the projectile in an intelligent way around a barrier. This is best for Sagas and groups that like the "I have 20 variations of Crystal Dart, one for each day of the week" style, encouraging players to invent spells and see Hermetic Magic as an inflexible (or, in the best, stone-hard) way of magic.
- We consider the opposite: by default, you should state the base Target parameter of the spell, and the decisions proposed by the description should be a little flexible, so we can choose to change the final targe7t. This is the best for a protagonistic vision of formulaic magic, where the stone-hard part of the spell are the general concept and the built-in parameters of the spell (Base; R; D; T).
- We consider that both cases exist, and there are formulaic spells that states the final target can't change. In the lack of clearness, we assume what is most flexible and/or better for the magus. The Crystal Dart is a case of lack of clearness. This stimulates protagonism AND allows more specific descriptions for spells. There's not much benefit, at first, for being more specific thar you need to be, but in most cases, this is a cool narrative feature.
Teleporting Projectile?
That said, Rego spells can teleport things, so it should not be that hard to ignore the barrier and set the dart directly to the face of your opponent, either using the ReTe guideline "Level 3: Control or move dirt in a very unusual fashion" or an analogy for the ReCo guidelines that transport the target using a set distance parameter (5 paces is Level 10, 50 paces is level 15, and so on). I would go with the second to be a safe bet. That would pass through most static barriers if you can perceive your final target, but most of these barrier ideas seems to be opaque, like metal globes. Arcane Connections can still be used for targeting someone in the range of the spell: if it is voice, all you have to do to reach the target is be heard - or, at least, the sound of your voice should "touch" the target, and the arcane connection should act to guide the effect to your opponent, even if the spell does not have "Arcane Connection" in range. However, you do need the arcane connection range if any other method could reach the target.
The "is ice clear enough?" question can be answered by the description of the Bridge of Frost - ReAq 30. It can depend upon caster's sigil, or decided at the casting as a cosmetic minor change, as long as I am aware of.
"In Cralian of Tremere’s version of this spell, the frost is so crystal clear that it can
hardly be seen. All of his spells are very subtle" (ArM5 - Core Rules, p.124)
A Note on Touch Rego
This is a Creo Auram 30 spell:
"WINGS OF THE SOARING WIND
R: Touch, D: Conc, T: Ind
Req: Rego
Generates a massive gust of air around you that supports and pushes you along through the air at speeds up to 40 miles per hour. Hovering in place is difficult, and requires a Finesse roll against an Ease Factor of 9 per round. It is dangerous for extended travel, because if you lose concentration, you are likely to sustain serious injuries (+15 damage, on average, although it may be more if the magus was very high up)." (ArM5 - Core Rules, p. 126)
We can notice, in this example, that you can still controll the effect as long as you are touching those winds (Range Touch) and concentrate (Duration Concentration). Of course, you should sense the gust, and we are considering that, since you're touching it, you're ok. We should consider thar "I know why I'm flying" is enough for the perception of the effect, otherwise a PeIm spell could destroy this touch sensation and make you lose control over your spell, unless you make it a stinky gust of air in the casting (so the smelling sense should be destroyed too for this to happen), or have another idea for it.
Note that PeIm spells are good options to hide the effects of enemies' spells and make them lose control of it. Or I'm completely wrong about it and you should ignore this 
If the Range was higher, like Voice, you could control the gust at distance, maybe even make someone fly as long as you concentrate and can perceive them.
The Art of Creo
There's not much to say about the obvious: Creo spells create things in a place within range. They don't usualy need any projectile. If you make a Creo spell work with a projectile, you should consider, obviously, the thing is a magic creation and needs to penetrate. It does not need an Aim roll, BUT with a Rego requisite you can both create and throw a stone. That can be Range Touch and then you use Rego to suggest the movement and throw the stone with an Aim Roll. Still a magic thing = needs to penetrate. The advantage of it is that... I honestly don't know, but isn't throwing fireballs cooler than just making things explode at voice range?
Ball of Abysmal Flame (CrIg 35) is voice range, but considers the projectile a cosmetic feature, so there's no Rego requisite. Like other offensive Creo spells, it needs to penetrate. Again, it's a magically created thing.
However, Creo alone don't give magi control over the thing. The ball follows to the target, and any surprise built in the way can stop the ball from hitting the target, right? Maybe. Mechanically, the cosmetic projectile feature should not make the spell less powerful. Ball of Abysmal Flame could be designed to just make the target explode. Why it was built in this way? A Magical Focus on Projectiles that the inventor wanted to keep? Well, that's not much relevant, but it is an idea. Both the personal magical preferences of the inventors and their casting sigils often fills the spells with features that makes them a bit less powerful or versatile as they could be, and I think this is healthy to the game. That said, I would stick with the option that the spell can be stopped.
See the CrIg 25 that follows:
"COAT OF FLAME
R: Voice, D: Diam, T: Ind
Req: Rego
The target is swathed in fire, and takes +5 damage every round while the spell is in effect. That is, the damage must be rolled against Soak twenty times during the spell.
The Rego requisite ensures that the fire does not spread.
(Base 5, +2 Voice, +1 Diam, +1 Rego requisite)" (ArM5 - Core Rules, p. 140)
In this case, we need a Rego requisite to stop the thing created to behave naturally. Rego needed to be added, because only Rego can make a thing behave in unnatural ways. However, we see that we do not need to keep concentration, since the suggestion "do not spread!" is built in the spell description. We can't use this spell to spread fire. That is not written, but this is the suggestion, and the spell does not allow the use of Aim Rolls or other Finesse uses to alter it. That said, it can be invented again with Aim rolls to provide more controll over the fire: the rego requisite is already there, but your group could consider that the requisite of a static suggestion is much less powerful than a flexible one, and add one more magnitude to the level - beyond the base +1 mag. for the requisite. (p. 115)
What House Flambeau has to say about it?
Let's call in the specialists.
"Only specially designed Rego spells can throw projectiles this way - the spell’s description must explicitly state that it can throw projectiles (these spells are higher magnitude than generic Rego spells). Spells that throw a projectile release control of it immediately after launching it" (HoH: Societates, p. 35)
This part is referring to one of the three techniques on Rego Aim magic: the case that I referred to "suggest the movement". The other two consist in guiding the projectile "all the way", and levitating the things above the target and leave the job to the gravity to finish.
So, the thing is that the use of Finesse to exert control over the Individual target (e.g.) is a complex thing, and that's why Ball of Abysmal Flame can't automatically hit the target like The Crystal Dart. So, the Rego requisite for make the "auto-hit the target" suggestion costs one magnitude on non-Rego spells. Creo and Muto examples here need to penetrate.
Pure Rego spells can hurl stones with Aim Rolls in a way that the caster loses control over the thing and trust the physics (basically what means an Aim Roll).
A lot of Rego Aim spells and guidelines are presented on HoH: Societates - House Flambeau.
The Fast Casting Option
So, we are assuming a successful Fast Cast roll for our opponent, right? Based on Quickness. If succeeded, it means the opponent reacts faster than the effect.
We are also providing that our opponent could determine the Form and think on a reasonable appropriate spell, in time.
"In general, a fast-cast defense with half the level of the attacking spell is enough to protect the maga herself or one other individual." (ArM5 - Core Rules, p. 83)
There's not much doubt now. Since Rego considers that:
1. The caster is concentrating to exert control over the projectile: The Quickness of the attacker was overcome by the Fast Casting roll of the defense, so the caster is not ''fast enough'' to exert control in time.
2. The spell has built-in ways to determine the behavior of the thing ("hit the target"): As Ball of Abysmal Flame is created with the same suggestion "hit the target" and could be barred by surprises, this option is also overcome by the Fast Casting roll of the defense.
While we are accepting that our opponent was fast enough, we just need to think about the guideline given by the rules: a fast-cast defense just need half the level of the attack to be sufficient. Of course this need to make some sense, like a fine globe of glass protects none from a big stone ball, even if the caster of the globe spent a lot of magnitudes on the duration and rises its levels. My final advice is to stick with the base level on doubt, but remember to keep your life simple - it is obvious that a wall of stone can stop fire, so you can just compare the levels. Remember that the level of the spell also represent the time that the maga spent studying, the experience on the arts and everything that involves acquiring sufficient casting totals to even fast-cast this, and all of that should be awarded.
Also consider the damage that the barrier would take, even if enough. The success on Fast-Casting, determining the Form, choosing an appropriate counter-spell and having sufficient levels to stop the attack should protect the maga, but maybe not the thing she just created.
And, finally:
"Fast-cast defenses against mundane threats work if the level of the spell is high
enough to neutralize the threat." (ArM5-Core Rules, p. 83)
Stick with this when Rego Aiming, cause the thing that arrives at the maga is a mundane moving thing.
Conclusion of the Metal Dome
For this conclusion, remember the points discussed:
- The Rego-Requisite spells that suggest "auto-hits" expects that the magus is perceiving his target AND final target - or else, Rego could not be used since 1) the magus must perceive his effect; 2) we need arcane connections to use Hermetic magic to affect something we can't perceive, and a Magus, at this moment, does not have one with his spells.
- Can you effectively perceive your final target AND the Indiviual target controlled by the spell (e.g. The Crystal Dart) to direct the projectile inside the dome? Rego counts with both of them, unless you have an Arcane Connection to the final target: in this case, you just need to see the stone to Muto-Rego it. Both need to be in range, since a spell can't exert control beyond the range.
- If the Crystal Dart wasn't Voice Range, and was Touch Range instead, it would be a Muto-Rego Aim spell that needs to penetrate. It could be aimed into the Metal Dome, yes.
- Would it have enough damage to pierce mountains just to hit the target of an Arcane Connection (in the case of Range Arcane Connection, of course)? Is it really reasonable just for keeping the "always hit" line? If the damage is important, the barrier should be considered.
- If a maga can see a hole in the dome (or make one with PeTe), big enough for a projectile she is planning to make AND to perceive the final target, they can cast the spell through it, since the final static built-in suggestion "hit the target" benefits by this new suggestion. Intellego spells could also be used for detections, like an InCo treating the Metal Sphere as Room target. This considers a straight and linear movement of the projectile, since natural movements are easier to reproduce with Rego magic. The base level of Rego Terram for controll the thing in a very unnatural fashion is just level 3, so I can think it works with the base 1 magnitude requisite of the Crystal Dart.
- One way or another, you mentioned in the Metal Sphere example that it was a fast-cast. See "The Fast Casting Options" above: the fast-cast defense success also depends on spell level.
Conclusion of the Stone Blob
- Does Coat of Flame still prevents the spread of the fire when the magus gets out of the voice range? Yes, it is a static suggestion. Is "auto-hit" a built-in static suggestion?
- Well, bellow the Coat of Flame quote, we come with an answer: a static suggestion is a +1 magnitude, and a flexible suggestion that allows magi to exert control over the Form COULD be a +2 magnitude requisite since it is more powerful. Does your spell have the +2 mag.? Let's consider that it has, or your troupe does not agree with additional magnitudes to keep it cheap. It is not really rellevant, since it is written in the text of the spell a static suggestion, anyway.
- So you "paid" for a Rego spell that you exert complete control over the thing and can guide the missile through the air as you wish? You're right, you can guide it freely. That's it.
- If your spell is like The Crystal Dart, you could just generate the projectile near the final target, not passing through the grogs, that's Voice range, right? I think it was not what you did, maybe because there was no Form material to work near the final target.
- There's the thing that the Level 3 Rego Terram guideline lets the magus exert controll over Terram in a very unnatural way, so, in theory, it should allow a lot of unnatural behavior of a projectile. If this is not in the description of the spell and it uses this guideline, you should think what makes the control over the thing so unnatural if isn't a complex missile control.
- The statement "always hits the target" is a static suggestion to the thing, and, as stated just above in the Metal Sphere conclusion, the projectile can be slightly guided to their final objective by the principles of Rego control over things, as long as the spontaneous suggestion directly benefits the final static built-in suggestion, but... it should be stated at the moment of the casting, since the spell don't let you control a missile, but controls it for you.
Yes, using player's lack of prudence against them is a dramatic technique that shoud be used with care and respect. "You forgot to say that the projectile should not hurt the grogs" is a good lesson about prudence with magic, and how magi should be careful because Magic does not make distinction between allies and enemies. That's why area damage is not always the answer if your loved one is your front-line. Even with all these good points, it requires some talking because it can be used as a way of punishing players and being an annoying matter in-game and out of it. It is fair, but a game should be fun too. Personally, I hate the trick "you forgot to say" and, as a storyteller, use this just in the moment that is NECESSARY for the CHARACTER to develop prudence, usually with player consent or very minor consequences that involves little or no issues with frustration. If the player commit mistakes, that just means he is human and not a real-world high-prudence magus.
I tried to do some analogies and argumentation that seems to make sense between each other, I can be wrong, deeply wrong and stupendously wrong. I am writing and searching this for a couple of hours, so the person who started is not the person who finished it. I am pretty sure there's some inconsistence, but treat them as me being imparcial and not-wanting-to-take-sides-on-your-problems, just here to talk about the game <3
YOU SHOULD NOT TAKE ANYTHING HERE AS A TRUTH JUST BECAUSE IT SEEMS A BIG TEXT, if this is not obvious. I used references, though. In the worst, trust them.
Sorry if it was too long. This is my first post (actually an answer) on this forum
