For a minor focus, I'd say no. But if she took a major focus in swords, I'd definitely let her use it to enchant an anti-mushroom sword.
Anyway, I like the character. Just two nitpicks:
Warping 70 is a typo, right?
Why's her Penetration specialty in Mentem when she has no Mentem skill whatsoever?
Both the warping and the penetration were typos. A result of me typing over a previous post to get the formatting right.
A member of the Confraternity of Roland would get his Focus bonus enchanting something into a sword, at least in my game. And so would she.
You are welcome to try and change my mind. I'd be open to rules quotes, arguments about game balance, and arguments on what would be more fun around the table.
My present opinion is that based on rules and game balance I like my interpretation and unbalanced rules can lead to things being less fun. In general I don't like taking power away from the players because people have fun with powerful characters but in this case I don't feel that I'm actually taking away.
I'm working on the first fifteen year period. The things I feel that it needs to address are
Getting Parma high enough to be useful
Getting something together to help with the covenant income (I'm planning on going further with rego craft magic)
and the issue of a longevity ritual. Taking a longevity ritual involves having any children that she wants to have first. Is Andrevea the sort of person who's going to go out and fight against the gift to get a spouse? No, she isn't, while she doesn't have low confidence she is extremely shy and I don't see her doing it. Now I have to figure out if she can trade some service for a well made ritual or if she'll make one herself to hold her over for a decade or two until she needs one with more skill. I'm going to throw some xp at corpus anyway. It isn't like that will be useless to her in the long run.
I'll provisionally do xp for 10 years leaving 20 seasons for lab work and I'll be prepared to change it when the other characters are up. then start putting up some lab work.
Parma Magica to 4 for 45xp
finesse to 6 for 36 xp (she has an affinity)
Magic Theory to 5 for 25 xp
single weapon to 6 for 30 xp
penetration 1 5 xp
Organization lore Order of Hermes 1 5xp
that's 146 xp on abilities leaving 154 for arts
Creo to 8 for 8
Muto to 6 for 11
[strike]corpus to 7 for 28[/strike](working with Bausus for initial Longevity ritual)
rego to 9 for 17
that's 64 leaving 90 for terram (which becomes 135 with the affinity) so terram 19 (11)
edit
Inetellego 5 for 15
Mentem 4 for 10
another 3 (which becomes 4 into terram, to make it 19 (15) it seems exceedingly reasonable that she could round up on xp at least twice during fifteen years of work. My characters tend to round up on their affinities about twice each year.
I just fit a unervingly large proportion of my "what I want to do with this character" notes into less than 20 seasons of labwork. I'll have to brainstorm some more stuff for her to do (a familiar might be a worthwhile project).
Here is a spell that Andrevea can knock out in one season. Some of the other spells are based on this one so I very much want to get this one right. I'm sure that I've seen its like before but I believe that it was for fourth edition. If any of you are aware of a relevant fifth edition example please let me know.
Is this the best way to generate totals?
Is it appropriate to include a method to defeat the spell? Is it believable? Would it be better balanced without one?
Are the extra magnitudes for increased Str and Dex appropriate? Should the amounts of added strength and dex be different?
Do I need a higher base level than "very unnatural movement "to have the sword more or less think for itself when it comes to attacking a target?
I would probably have prefered Str 0 and Dex 0, but using Finesse directly as skill (not Finesse-3).
A spell like that is always gonna be pretty ad-hoc, but what you have there looks good to me.
Would that be as a level 15 spell? I added a magnitude each for the strength and the dexterity (largely because her lab total allowed it and it would be more controversial 
).
The craft magic rules in Covenants made the ease factors for finesse rolls three higher than the ease factors for the equivalent craft skill. This is what inspired the use of finesse -3.
By your estimation my spell is under-powered. Sanctaphrax thinks that it is OK. Here's an alternate version that takes a more liberal view of the needed guidelines
I made the attack equal to finesse, I removed the ability to stop the spell, and I equalized the effects of the extra magnitude on strength and dex.
Is it preferable to the original? Could I have done +6 dex and +0 strength instead of +3 to each?
I agree that a spell like this is going to be pretty ad-hoc. All the more reason to make my example spell well thought out.
Good point. 1 magnitude for Dex +2, but also only 1 magnitude for Str +4? Dex is more useful/powerful, but this feels odd, somehow.
I assumed so. Finesse -3 could work. If it's a purely magical manipulation, I'd go with Finesse though. Rego can move the sword in ways and angles no mortal swordsman could possibly match.
Not sure if I agree with 'under-powered'. But there are a few details I might like to nit-pick.
I think I prefer this version. Fun thing is, in the 3rd edition I would've probably prefered the first version. But this version seems more consistent (IMAO).
Absolutely.
I'd maybe look at the Unseen Arm and Unseen Porter spells (and any other published variants) for scaling and ideas.
For instance, one magnitude seems to be enough to go from negligible Strength to Strength +5.
If the spell is under the magus' control, what about simply using a normal aiming roll, modified by the weapons stats?
I'd also consider making a note as to whether or not the spell is resistible or not. If the whole blade is affected by magic, yes? If the magic is "wielding" the weapon and only affecting the hilt, no? Maybe there are "regular" and "Vilano" versions?
What about the possibilities of casting requisites, similar to the Unseen spells above, in case you end up with a wooden sword (perhaps via a Muto spell?)?
I agree that Finesse-3 is appropriate. That's because, as with Craft magic, you are using one Ability to substitute for many other Abilities. If the magus has Single Weapon, he can use that instead (and I would remove the -1 penalty), but without the proper Ability he must use Finesse at a penalty. This seems most in the spirit of the rules.
I was glad to see a defeat condition in the spell, as a GM would otherwise have to decide how such a sword could be stopped. Since this is a question every GM will have, let's address it up front.
When I first saw the Str and Dex I thought them high, but once I realized you had dedicated a magnitude for each, that seemed reasonable.
Simply eyeballing the spell, it does not seem too powerful. It's not unusual for a level 30 spell to simply slay a target; to have a level 25 spell which attacks with a sword -- something many NPCs will be able to defend against, and against which they have armor -- does not seem exploitive to me.
The point of the spell is to send the sword off to do its thing and leave the caster free to other things. Also I was serious about the "as if wielded by a person" part.
Years ago I was playing a magus with a focus in oak trees. I developed a spell to move groups of huge logs from the trunks of ancient trees according to his mental control. I wanted to use it to create structures, but it was also a way to wack things with weapons that out-weighed rhinoceroses, a way to fly the character or even fly groups of people, and a myriad of other uses. Regardless of the level it wasn't a fun spell for a game.
Insubstantial Duelist takes some inspiration from unseen porter in that it acts like a weapon wielded like a person and thus avoid the issue of being the one spell that can be used for everything that my "dozen tree trunks flying around like superman and whirling like Cuisinart blades" had this also relates to
I think to get around MR this spell would need to "drop" the sword just before it hits. I think that the sort of awareness required of the spell to pull that off is beyond the guideline I used, perhaps even impossible by a single normal spell in that it involves too much thinking.
I see resisted as kind of the default. Would you prefer that I make specific note of the fact that it is resisted? Obviously it would be simple to do and not change the meaning of the spell but I fear that the mention would cause more confusion than it resolves as the reader wonders why the note is there at all.
If Andrevea creates a spell that also functions for sticks then I don't think it is proper to apply her magical focus to the lab total in creation of the spell. So while it's a sensible idea for a spell, that's not what she'd want to do. In the specific instance where a metal sword is changed to wood by a muto terram spell I think that the wooden sword could still be targeted by terram magic in the same way that a person under the effects of cloak of black feathers can still be targeted by corpus magic. The essential nature of the sword can generally not be changed by magic, the target just gains an additional nature for the spell's duration (per the discussion in Mystery Cults page 22).
Upon reflection I like the idea of the sword being knocked aside for a round then getting back up and fighting again whether used by a PC or NPC that sounds like it would be cooler at the table than getting knocked to the ground and ending the spell for some unexplained reason.
I also like using equal values for the two attributes.
+6 dex is a superhuman attribute and I don't feel that this comfortably fits into a single extra magnitude.
+3 is the level for change difficulty for a "single level" and shows up all over the place. I can accept that attribute for sword fighting are different than attributes for unseen portering.
So here is a draft of the spell that I'm leaning towards
wait, hold on I see Dr Comics has responded as well
The defeat condition, Yeah that's a hard one. the sword getting back up is pretty badass and would be fun but it could be frustrating as well. But I'm still leaning by a tiny amount towards taking the defeat condition out. It isn't easy to justify from a theory standpoint and I don't think that the spell is overpowered as is.
I took your suggestion on the single weapon skill, But I note that this is not a spell that is concentrate to control, it is fire the spell off and it will do its thing with no direction needed.
With the idea that +3 represents one difficulty level for skill checks one might argue that +5 str represents one health level. I don't buy the argument.
OK here's the new draft:
My mind is fickle and I welcome you to change it by persuasive arguments.
I agree that making the defeat condition temporary is the right call. Players working together could knock the sword down and then use the free round to throw it in a barrel or something, which I like. At first, since the defeat was only for a round, a +15 damage seemed excessively high to me; I would much rather it was the equivalent of a Heavy Wound instead. But then I realized that there is no mention in the spell of the sword having a Soak or anything, and a Soak of 4-5 would reduce the +15 damage to about what I wanted to set it at. If you intend the sword to have a Soak, I would reduce the damage necessary to temporarily stop the sword.
Otherwise, it looks good to me, and I agree that it needs to penetrate Magic Resistance. My hunch is your maga is going to have ways to make that happen, which I look forward to seeing.
The one thing that seems strange is that normally when you stop directly controlling a spell it cannot adjust in such ways. For example, using telekinesis you might set something in motion while floating a pace above the ground and that motion could continue, but it would not be able to follow a road while doing so without you directing it. But here the spell seems to go quite against what appears to be the standard for non-mobile things. (Giving the semblance of life to a statue and similar seems to behave a little differently and require a higher base.) I would think you would need an Intellego requisite for that reason.
Can you even use Finesse passively?
I support the view that the weapon needs some way being quasi-sentient. It's not the finesse as such, it's that the weapon will continue to strip in the manner controlled by the caster. Animation spells normally have a requisite, animal or Mentem. Wouldn't that apply here too?
Can you find me an example of an animation spell with a mentem requisite please? I'd like to see one to crib from but none come to mind.
There is a guideline under Rego Herbam that had slipped my mind
That's pretty much exactly what I want and it doesn't have a requisite. The example from this spell in the core book is the treacherous spear. The treacherous spear targets a weapon being held by someone and uses that person's combat scores to attack. The treacherous spear is dispelled by damaging the weapon. It is a bit different from my spell but it does show using a total derived from someplace else while the caster is not concentrating
Now I taking that guideline to terram I imagine that I''ll have to add two magnitudes for metal rather than wood. This annoys me from a game balance point of view but it certainly seems that the intention of the guidelines is to make doing things with metal more difficult than doing things with dirt even if it is weird in this circumstance. On the other hand unseen arm and unseen porter both ignore this restriction.
Do you think that using the caster's finesse makes as much sense with this guideline as using the combat statistics of the sword's last wielder? Is "as sensible or more sensible than this particular goofy legacy spell" too low a target to shoot for?
Hmm, from cannon I don't think I can. I asked for help on the forums a while ago for granting speech to stone with one spell and then sight with another url=https://forum.atlas-games.com/t/spells-for-moderation-sentience-and-voice-to-stone/8935/3[/url] and used a base 4 guideline and then added a Corpus requisite. I thought the two effects were better separated for my purposes, and it's not really a match to what you're proposing, so take it or leave it. 
.
I take your point on the ReHe guideline and also for the circumstances; suggest that giving a form of intelligence to a plant should be easier, or at least different to doing that with stone as plants are already at least living. Inanimate dead substance has to change two fundamental properties, where a plant only needs to alter one (?).
I also thought the Treacherous Spear doesn't quite have true intellect to act on it's own to attack - it thrashes around wildly but doesn't really know to attack somebody in the same manner a caster would when using Finesse and controlling a spell directly. Were it to animate and continue to strike them in a dynamic manner I think it would be a better match. That's where I think there is a subtle difference. Basic animation is perhaps different to a plant that can react to stimulus outside some very simple pre-programmed instructions.
I remember a forum post from a while ago where the discussion talked about the difference between animating a statue with (just using ReTe) to move vs making it act like a bear (ReTe/An), and at what point is a requisite needed. From memory the difference was the degree of autonomy it needed and how broad it's range of abilities would be (or at least hat's what i took away from it).
I went digging and found Stir the Slumbering Tree, Ars 135, MuHe/Me, which has a Mentem req. And Command of the Forest, ReHe/Me, MoH p40, which also requires Mentem, and is using the same baseline level 10 that you've mentioned.