Darned exceptions popping up everywhere. Hard to keep the rules straight sometimes. 
Yes to both.
Works. But:
- This is a constant item. Why a broach? Better to have, say, a cloak be enchanted, at range personal. Your design is strange.
- This is a constant item, meaning it'll cause warping. Which ain't a bad thing, as it allows all kind of descriptions of SFX and strange bits about the item
- This is a constant item that doesn't need to be "on" all the time, meaning you could get with Sun duration and 24 uses per day, for the same cost, which would allow you to protect other people.
Yet another example on how Constant Items are the poor child of Item Creation 
Works finely.
Hard to say. It seems that, usually, but not always, there's a continuum like:
Spont Magic more flexible than Formulaics more flexible than Items and Rituals, with power going the reverse.
I'd make it work like the spell, but also write it clearly so that it requires a finesse roll to tell the illusion's quality (like the spell again, but this is often forgotten, so emphasize it).
I think it is washed up in the abstract. IIRC, I took no exposure for Arachné.
Errr... No.
The rules are just a mess, with a clear rule (you need the gift) and many, many exceptions, Failed Apprentice being one of them.
Going strictly by your pow, even average, totally mundane grogs trained in MT would be a great boon to a magus: With just leadership 2, any magus could be helped by 3 grogs with Int 0, MT 4, boosting his lab totals by 12. Clearly, this is not the case 
To be fair, there are a lot of things that could be done easily in Ars Magica that aren't. At least there is some consistancy of the idea of not using grogs as lab assistants, given that most magi would probably think they would need to be trained in latin and artes liberales in addition to magic theory to be of any use, plus if they start using grogs as lab assistants who would wash their clothes? Much easier at hat point to train them as scribes with MT 1 and let them copy books...
Believe it or not, 5th edition is much better than previous editions at these odd exceptions being pointed out in unusual places. But the key to the game is remembering it is about telling stories, and the rules a troupe goes with affects the stories a lot. In this saga, Marko doesn't enforce requisites on Reco Corpus "teleporting", for lack of a better word. But in other sagas and even within another book (Houses of Hermes: Societates) says that requisites do apply to what the magus is wearing/casting. It's not a big deal for a 15th level spell to have requisites, but it becomes a huge big deal with a 35th level spell. That's just one example.
So here are some of my thoughts. As noted above, there is a clear rule that says that you need the Gift to help. Then there's the Failed Apprentice status. It states:
This says that you can serve as a "laboratory assistant." Does that imply you get to add your Magic Theory to the magus's Lab Total? Unclear. The only other time in the rules that the term "laboratory assistant" is used is when referring to familiars. There it notes:
This seems to imply that when they use the term "laboratory assistant, they do mean an assistant that can provide tangible aid (i.e., a bonus to your lab total). After all, a Familiar doesn't have the Gift either. Perhaps the Failed Apprentice is another exception to the Gifted requirement.
Of course then Covenants uses the term "laboratory assistant" to clearly describe a grog assistant, who should not be able to add MT to LT. (See, Covenants, page 53, second column.)
Then Covenants notes under the Outfitting Virtue "Assistant" that the assistant who adds to LT must be Gifted, while noting that a "Servant" simply adds to Safety:
As Fixer said, these particular rules are a mess.
Ultimately it's up to Marko, but based on all the evidence I would tend to lean toward allowing an assisting Failed Apprentice to add her Magic Theory to a maga's Lab Total. In particular, I would interpret the description on page 42 of the main rules as carving out a second exception to the rule that an assistant has to he Gifted. That's not where I expected to end up. But after reviewing the rules, that's where I come down.
In the end, I don't think it's a big deal. After all, how many failed apprentices who lost the Gift are there out there? It's not like they're growing on trees and could be hired on demand.
You're overthinking it.
The Failed Apprentice is an exception to the rule, as the Virtue itself states. As is a familiar, because that's the point of the familiar and you spend a lot of vis to get a familiar.
In all other cases, again as stated in Covenants, a mundane assistant counts as a Servant (lab virtue). And whether the Servant actually knows Magic Theory is immaterial, as the bonus for Servant is only their Intelligence/2 as a bonus to Safety (not GQ), +1 Aesthetics and +1 Mentem.
I'm a patent attorney. I can't help but overthink things ...
Ok. You're underthinking it, then. 
Think about the shape of the Order, if grogs could actually help in the lab. You'd see not one, but several grogs (who aren't Failed Apprentices) with Magic Theory scores in the 6-8 range at every covenant, at least one per magus, if not more. And they'd be smart; capable of adding 2 magnitudes to a magus alone, and if the magus's leadership was high enough 4 or 6 magnitudes would be almost trivial on a particular project. It would be a huge change to the setting that must have existed before the players started playing.
When you think the rules allow you to do something too easily, thinking about the implications of the setting as a whole, and also ask yourself the question, why is your character to figure this out? Nine times out of ten, it's a (selective) reading of the rules.
That being said, it can be trivial to get some truly impressive lab (and casting) totals in specific areas based on affinities, puissant Arts and magical foci. Having mundanes add their Int+Magic Theory makes those capabilities available to just about every magus, but it applies more broadly to every technique and form, essentially.
Nope, this is not correct.
In a cycle you have X number of years (typically 5-10 I think). Within a year, you have 4 seasons. Of those 4 seasons, you have various options that you may undertake.
They are:
- Advancement (up to 3 seasons worth) - Gain 10 XP
- Work in Lab (up to 3 seasons worth) - Gain Exposure XP
- Earn season of Vis wages (up to 1 season)
- Something else - Gain exposure XP
So, to answer your question, you need to determine how many seasons you are spending doing lab work, how many seasons you are doing âadvancementâ and how many seasons you are doing the other things. Keeping into account the limitations listed above.
So, in a 7-year cycle, you have a total of 7x4= 28 seasons. One of those seasons is wasted per the house rules (1 season per cycle is wasted). Of the 27 seasons left, up to 21 of them (7x3) may be spent on âadvancementâ [gaining 10 XP]. Up to 21 of them (again 7x3) may be spent doing lab work [gaining exposure XP]. And up to 7 of them (7x1) may be spent earning extra Vis wages [giving 3 pawns per season of any Form].
- You do earn Exposure from Lab Work, as it is a separate isolated activity removed from the abstract.
- For Experimentation, the bonus is a Simple Doe, but there is also a roll for Extraordinary Results that I must enforce
- Failed Apprentice also implies that you had the Gift at some point and you had been an apprentice to a magus; but for some reasom or somehow you gift became damaged or flawed or something. You have a partial or residue gift.
- There is one more exception for Verditius magi. Forge Companions add +1 for every five points of their own Craft ability if it applies to the project.
Actually while there are many places where it states that a gifted assistant with magic theory can add to your lab total, but nowhere does it state that the person has to be gifted. Note under Magic Theory where it states "Anyoe can learn Magic Theory, if they have access to a teacher or a book, but it is little use to those without the gift."
Speaking o making stories interesting, what would anyone think of letting magic theory add to craft magic (for wonderous items only, not mundane excellent items), and further allowing that those touched by magic can act as lab assistants? This would encourage Donna to act as a lab assistant (and interact more with magi) in order to get experience in Magic theory... If not no big deal, I thought I would at least throw out the idea...
I just looked it up, and though it is true that the original wording in RAW can be construed ambiguously, let us choose to keep it reasonable. Lab Assistants that contribute yo your Lab Total are (for the most part) limited to the Gifted, Familiars, and those with the Failed Apprentice Virtue. Forge Companions are a different situation.
While this is true, it is never explicitly stated that someone without the Gift can help with Magic Theory, except for the Failed Apprentice Virtue. Further, in the Help in the Laboratory section on page 103, the text specifically says "Anyone who has The Gift and a score of at least one in Magic Theory..." And is a pretty big deal in that sentence, meaning you must meet both conditions of being Gifted and having a score in Magic Theory. If the Failed Apprentice virtue didn't explicitly state that they can help in the lab, then they could not, by virtue of that single sentence.
It says that anyone with both can, yes I am aware of that. What it does not say is that those without cannot. It could be that, similar to the rules for midwifery, a certain degree of hands on experience is necessary without some non-theoretical training in the processes, but that could include those who are gifed, those who were gifted (failed apprentices), those bound to teh gifted (familiars) and perhaps non-gifted with certain connections that make sense (craft mages assisting verditius, shapeshifters helping with MuCo(An) research, Crafter's healers helping with healing magic...) or indeed if read technically, it does not forbid anyone from acting as an assistant, it imply grants a lab bonus to that particular group... perhaps the ungifted add 1/2 magic theory and not intelligence, similar to craft assistants, or perhaps only if the magus has the appropriate virtue to use 'ordinary' assistants... there are a lot of places this sentence allows, the only thing it actually specifies is that those who have both can add to lab totals.
ahem
Official Ruling of the SG
Lab Assistants that contribute to your Lab Total are, for the most part, limited to the Gifted, Familiars, and those with the Failed Apprentice Virtue.
So mote it be...
(Big Red Hammer[sup]TM[/sup])
Good grief, I go away for a bank holiday weekend leaving an innocent question, and come back to nearly a full scale riot! 
Ah, bother, I'd forgotten about the constant item warping. It's a brooch because I'm good at jewellery; and it seemed more fitting for its intended purpose, which is as a "dowry" of sorts, to be given to Isolde's father as I start play to soften the blow of marriage to a Gifted individual. Warping would not be a good idea. 
The trouble with Sun is what happens if you want to get wet (to, say, have a bath) if you've put the brooch on since the last sunrise/set. I guess what I really want is an effect that acts exactly as long as the item is being worn. Does such a duration exist?
Good plan. Will do. (I'd forgotten I'd need Finesse. I don't have any. Oops)
Cool.
Okie-doke. I'll bear that in mind.
It seems to me that if you have duration concentration and the item maintains concentration then when you remove the item it would stop providing concentration. If putting t on is how you activate it then hat keeps it pretty simple...
Ok, I've worked out how I want to do the brooch and invisibility pendant (which may become a ring). I'm now working on an item with Purity before the Noble's Court, Aura of Ennobled Presence and possible Disguise of the Transformed Image in it. I may well also make the object my Talisman.
Meantime: I Experimented on the invisibility pendant, rolling a Side Effect - Minor Flaw: "For example, a spell that allows you to communicate with animals causes you to retain some of the animal's speech patterns for a time after the spell ends". Marko, would you care to suggest a Minor Flaw for this invisibility device?
it inflicts a -1 penalty due to inherit clumsiness because you cannot see or sense your hands?
maybe?
Hmm. That seems more like a problem with the spell than with the device. I had a thought overnight - how about it leaves your left thumb (or another fixed small body part; nose, say) visible? At your call, perhaps you can eliminate this by wearing something over it (so, no problem if you have gloves on).
Or it makes your footsteps squeak slightly - a difficulty 9 (off the top of my head) to hear?