Two questions:
The base Target for an Ignem spell is a large campfire (ArM pg 139), which does not produce the same amount of light as a brightly sunny day. Is the amount of light transported by the spell equivalent to that of a campfire, or should the magnitude be increased to accommodate the larger "size" of the target?
When you talk about the invested device bringing breezes and/or images, you are speaking of additional effects, right?
That's kind of mixing guidelines, and even if the limitations of the CrIg guidelines could be applied here, look at Lamp Without Flame.
That's a good question. The Ignem guidelines do not clearly define what is the base Individual is for light. Jonathan is right, though, to point to Lamp Without Flame as an example.
I based my spell on the amount of light and heat that would come through a normal-sized door or window, as that seemed reasonable. A size magnitude might need to be added if he was trying for a castle gate, I think. Anything larger than 10 square paces (10'x10' or 3x3 paces).
Yes. It would be either as separate items/windows (for lesser enchantments), or multiple effects (requiring a full Invested Item).
For an item to provide lab improvements, there's actually very little reason to go for the three effects into a single item, aside from not needing to double the level of the effect. And the coolness factor of opening a window and seeing and hearing what goes on outside, while feeling the breeze and the heat of the sun on your face, all at the same time. 
Filling the Lamp (ReAq 10)
R: Touch, D: Mom, T:Part
By touching a lamp and another recipient containing oil, instantly teleports enough oil to fill the lamp. This spell can also be used for other objects and liquids, and a casting requisite appropriate to the liquid may be required.
(Base 4, +1 Touch, +1 Part)
Notes:
IMO, all of your spells should be in the ReTe area, relying on requisites to transport the individual object.
It aligns everything to one set of guidelines, as there really aren't any guidelines for transporting liquids, just guidelines which are based on transporting things. And then those things are governed by requisites, IMO. With the exceoption of Rego Corpus, there aren't any guidelines that exist that do this, and you're attempting to replicate Hermes Portal on a smaller scale...
There are no guidelines to transport anything but Corpus and Terram only because none of the book writers of Ars Magica ever tought of a useful spell in other Forms. Even the Terram guidelines were only introduced in MoH, and are explicitly based on the Corpus guidelines. So I'm essentially doing the same and extrapolating to other Forms.
As for the original research, Petronius is trying to develop an overall theory that ReTe portals would not require casting requisites. I've already alluded that his first step would be developping spells for as many Forms as possible, to study the specifics of teleportation of these Forms, in order to compare that with the workings of a portal. After all, if there are no casting requisites for portals, there must be a reason? So studying different Forms independently of ReTe seems like a valid research avenue for the early stages of his work.
Note that there is absolutely no mechanical advantage of proceeding this way, as it requires him to develop additional Forms instead of just concentrating in Terram (and to a lesser degree Corpus).
And diversifying his research outside of Terram is also much less boring, story-wise.
So, if I understand correctly...he's discovering new guidelines to understand why Hermes Portal works the way it does?
Something like that. He's basically deconstructing translocation effects into individual Forms, looking for the ways in which they could be recombined into a solid theory of how a portal can be built to recombine them all into a unique Terram effect linking two locations in space.
(Feels like pseudo-scientific Star Trek geek-speak! 
)
Just realized something regarding this spell, as I was re-reading the description for Seven-League Stride. It does not need to use Arcane Connection range, even if it uses an Arcane Connection to determine the destination. The Base determines the transportation range. The big 'R' Range is the range to the source of the transport.
This means that the spell should be R:Touch, saving 3 magnitudes. So the lowest-level version would be level 10 (using Base 4 for a 5 pace transport range). Then Petronius could increase the range by using Base 5 (50 paces range) for a suite of level 15 spell in Ignem, Aquam, Auram and Imaginem. Much easier to invent. Also makes it possible to increase the duration without moving the level too high.
Am I completely off-base here?
The Target (light) exists at the other doorway. You're transporting the light hitting that location (the remote location) to the present location which you indicate. The Target is at Arcane Connection Range, and you are moving it locally.
Seven-League Stride and The Leap of Homecoming are transporting you to the Arcane Location (which is part of the base level effect).
Maris, NPC in Bibracte has a spell that can transport someone to her, if she has an AC to them. The Range to Target is: Arc. Your first understanding is correct.
PS: I deleted the double post...I also have a totally hidden post that you'll see if you do a reply to the forum. Hate the forum today.
The Target (light) exists at the other doorway. You're transporting the light hitting that location (the remote location) to the present location which you indicate. The Target is at Arcane Connection Range, and you are moving it locally.
Seven-League Stride and The Leap of Homecoming are transporting you to the Arcane Location (which is part of the base level effect).
Maris, NPC in Bibracte has a spell that can transport someone to her, if she has an AC to them. The Range to Target is: Arc. Your first understanding is correct.
PS: I hate the forum right now.
You have it backward. I am transporting the light going through the local door (that I am touching) to the remote door (distant location sense through the Arcane Connection). I'm basically pushing, not pulling.
This is the same thing as a Touch-ranged Seven-League Stride.
Probably needs the following edit...
By casting this spell on a touched doorway, window or other similar opening, while holding an arcane connection to a distant but similar opening, the caster transports natural light and heat of the sun shining at the local (touched?) doorway to [strike]through[/strike] the distant doorway
You're right, my spell description was the one in error. My bad, I'll correct it. 
I get it that once corrected I would be right to use R:Touch?
Yes! 
The new edited version:
Portal of Abundant Sunlight (ReIg 20)
R:Touch, D:Sun, T:Ind
By casting this spell on a touched doorway, window or other similar opening, while holding an arcane connection to a distant but similar opening, the caster transports the natural light and heat (such as those from the sun) shining at the local doorway to the distant doorway (provided that it is within 50 paces). Someone standing on the other side of the local doorway can still clearly see the scenery outside of it (as the Imaginem species are unaffected), but there is simply no light nor heat coming through the doorway to illuminate what is on this side of the doorway. Any increase or decrease in the light or heat at the local doorway is reflected in the amount of light and heat shining out of the distant doorway. This may be due to the position of the sun, clouds obscuring it, or even the shadow of an object or creature blocking parts of the local doorway.
(Base 5, +1 Touch, +2 Sun)
Notes:
I had an idea regarding Petronius' avenues of research for portals. It is linked to the concept of arcane connections.
Linking two points in space to form a portal between them looks, at first glance, quite similar to Opening the Intangible Tunnel, which uses an arcane connection to channel magical energies to a distant target.
What if, as part of his original research, Petronius was to investigate the creation of a permanent arcane connection between two locations, so establishing a portal between the two becomes much easier? In a way, combine something like Opening the Intangible Tunnel with something like Wizard's Leap. The goal would be to apply the range-extending properties of a Tunnel to the Base guideline (which determines the distance of the translocation effect) instead of the Range of the spell.
Because the way I look at it, translocation spell have basically 2 ranges: the range to the target being translocated and the range of the translocation. The second one has simply been linked to a guideline because spells only have one Range parameter, so it was simpler from the standpoint of game mechanics.
This needs not be perfect. Perhaps establishing such a connection between two locations require many similarities between the locations (physical features), which would explain why one cannot simply establish a connection between any two locations. So a small circle of oak trees with a small stream running through it might only be linked to another location with the same features.
I was thinking that establishing such a connection between the locations could be a CrVi ritual. This would allow the effect to be permanent, but also fits in Petronius' Mercurian lineage. Thereafter, two point this linked could be traversed by opening a temporary portal spell using a lower guideline than that suggested by Hermes Portal (which is 6 magnitudes higher than the equivalent Terram transportation guidelines and 4 magnitudes higher than the Corpus guidelines). This would allow for a Concentration-duration portal spell that does not have to be a ritual because of the high level. It could still be a pretty high-level spell (up to level 50). The ritual to establish the connection between the two locations might be of an even higher level -- it's advantages over a traditional Hermes Portal would be that its permanent.
So basically what I'm proposing here is a rationale for Petronius to investigate effects relating to ReVi and CrVi as part of his original research. The ReVi effects similar to Opening the Intangible Tunnel might be covered by his Minor Magical Focus with Portals, but the resulting investigation to create a connection using CrVi would not be covered by his focus, so it would be harder to develop. And the CrVi connection could be useless for anything else but establishing a portal, so it would have no impact on spells that use R:Arc. It might be the start of such research, though, to broaden the usefulness of the new guideline.
((I hope this idea that I have is clear enough... let me know if I need to clarify or explain further.))
What I'm looking for are comments on this idea, and whether the troupe thinks that this would be a valid avenue of original research for Petronius' portal research.