I don't have a compilation of all the Peripheral Code rulings, so there might be something relevant that I am not aware of. If so, please enlighten me.
My players are about to be caught up in events they have been hearing about from Redcaps.
It started where Mage A is kicked out of his House for being involved in a series of disasters that made a lot of Magi mad, including his House Primus. Having the Flaw Judged Unfairly meant more than his fair share of blame fell upon Mage A.
Mage A takes his mostly trained apprentice, and calls in markers to get Mage B of House ex Miscellanea to shelter them and sponsor them for House ex Miscellanea membership.
Unfortunately for Mage A, irate Mage C has tracked him down, demanding apologies and recompense for the disasters, which due to egos and misunderstanding leads to Wizard War.
Mage B's shelter can't protect Mage A from Wizard War, so Mage A left his apprentice with Mage B and tried to outrun Mage C. On the final night of the Wizard War's month duration, Mage C managed to corner and kill Mage A.
Mage C determined to get the recompense they think they are owed (which was what the Wizard War was about), comes looking for all of Mage A's possessions. Including the apprentice. Before Mage C arrives, ex Misc Mage B (for reasons of their own) has initiated the apprentice into Mage B's lineage mystery and claims this means the apprentice is now a fully trained mage rather than an apprentice. At least, once the former apprentice's Oath is witnessed by other Magi (ie the player Covenant).
So mage B and the (former?) apprentice show up on the players' Covenant doorstep with mage C a few paces behind.
Is there anything in the Peripheral Code that applies in this situation?
There is Apprentices p.38ff Legal Issues about the dubious legality of fosterage and the easy way to trade/transfer apprentices from one magus to the other.
If A has unequivocally and provably transferred the apprentice to B before dying, C - unless he's a Bonisagus - can do little about it but enter another wizard's war, or perhaps a certamen at some Tribunals and times.
That B has initiated the apprentice does by itself not prove the transfer: it could have been done in dubious fosterage as well.
Wizard War only lets a magus slay / deprive another magus of magical power for its duration. Unlike a March, victory does not override the normal rules of inheritance, or convey any rights in property not already seized. So, if the apprentice hasn't been legally transferred to B, then as a possession they will now belong to A's heir (which will vary by Tribunal, as well as whether A left a will). If C tries to take them, they will be attempting to deprive that heir of magical power.
I'd say that if the apprentice has sworn the Oath, then he's a full magus; magus B as the sole witness is quite sufficient.
If he swore the Oath less than fully trained, then magus B might face some legal censure (it's a Low Crime to train an apprentice improperly in many Tribunals) but that's about it. Of course, magus C can always resort to Wizard War against magus B, against the former apprentice etc., though that's likely to get him into trouble with the Tribunal eventually.
This, very much this. While the victory in a WW would let MC claim anything MA had on him at the time, it does not give him ownership of everything that MA owned. If he could not capture it and hold it during the WW he is out of luck.
Hiding in a Covenant, yours or an other that lets you stay, actually is a very valid defense against/during a WW. It greatly limits what the other Magi is able to do. Any action he takes that would be a violation of the Code against Magi he has not declared a WW against are still a violation of the Code. MC is likely to do something that gets him marched unless he is very careful and subtle in his effects.
MC could threaten to declare a WW on every on every Magi in MB's Covenant if they did not expel MA. What the Covenant would do varies all over the board, from tossing MA and everything he brought with him out, to tossing MA out but letting him leave all his stuff with MB, to telling MC to "bring it peon".
Finally MC going around as he is will also most likely very quickly find that every Covenant he attempts to visit will tell him "your mother was a hamster and your father smells of elderberries". He is being a dangerous nuisance, so should not be given a chance to interact with them or scout their defenses.
Possible Side Effect: There are actually quite a few Flambeau Magi would happily get involved in a WW with MC if he becomes enough of a nuisance. The white knight types, the ones who just like to fight, and the ones who seek to purge dangerous elements from the Order. Heck many of them enjoy it so much that they will swear an Oath that "if someone slays Magi X through WW, they will declare WW on that Magi" for little beyond the chance to get involved in a good fight. Flambeau do like a good fight.
I think "any action" is a significant overstatement - yes, it's relatively easy to do something that is violation of the code when attacking a magus hiding in their covenant (e.g. causing collateral damage that affects the magical power of the other magi, injuring / killing covenmates trying to expel you for trespass), that doesn't mean that any action is automatically going to be a breach - if you're able to successfully sneak in and kill them / have them killed (by e.g. a mind-controlled / bribed servant) without affecting the other magi then that's likely to be fine.
We have HoH:TL p.47: "The core interpretation is that the recipient of the declaration has a lunar mouth to prepare and then the war lasts for a lunar month. During a Wizard War both parties can attack each other’s life and property without fear of prosecution. However, they still may be held to account for any collateral damage." There is tmk no stipulation, that a Wizard's War does end immediately after the death of one of the contestants. Which kinds of looting are covered by "attack each other's property" may well be decided at a Tribunal - such as taking away an otherwise not legally claimed apprentice.
in order for magus C to have any claim on the apprentice he has to grab the apprentice durring the time frame of the wizard war already declared
C may not violate B's sanctum in order to garb the apprentice
Whether the apprentice has been legitimately inherited by anyone
I would think the wizard war would take precedence until it ands- after all this would allow Magus C to raid Magus A's sanctum and lab post mortem for the remainder of the wizard war. If the apprentice is not grabbed during that time then normal rules of inheritance of the apprentice would apply, including the possibility that the apprentice would be considered abandoned. If both A and C claim the apprentice as abandoned it would probably be decided by a Guernicus, and for C to claim the apprentice as settlement of a debt then the debt has to be recognized by the tribunal.
After the death of Magus A, his apprentice could end up being "free" and if so then magus B could in principle offer the apprentice to "teach him magic" at which point the apprentice is fully and legally the apprentice of magus B, since the apprentice was for a brief moment just a normal Gifted "kid" with no master, a situation which magus B "kindly" offered to remedy.
I have to agree with ezzelino here that if the apprentice has passed a legal guantlet, sworn the oath and been taught the secret of parma magica then that apprentice is now a fully fledged magus with all the legal protection that entails and as such cannot be "claimed" by anyone anymore. Though it is possible that magus B could be charged for the theft.
Yes - if you kill your opponent, you've got the rest of the month to nick their stuff. But the scenario above had the victim killed on the last day of the war, and the apprentice show up after it was over.
I'd missed the sworn the Oath bit. But yes, if this has happened, then they're (legally) a full Magus, not property. Convincing C of that is the trick, of course - but if they're not convinced, they're on the hook for deprivation of magical power, with a truly humiliating punishment if convicted (a season for a season seems just, and what magus wants to do service to someone they think of as still an apprentice?)
This does raise interesting issues about how the Order deals with swearing other people's apprentices to the Code. They clearly permit it when a master keeps an apprentice too long, but what about when someone swoops in and does it too early? Is the oath invalid (and the apprentice still an apprentice), or is it just a serious deprivation of magical power? I'd incline towards the latter, unless a Tribunal has set formalities around the oath which were not complied with.
Assuming that a sworn oath can be declared invalid, and a formerly freshly gauntleted magus can be reduced back to being an apprentice, what about the fact that the now-apprentice-formerly-magus knows the parma magica?
This is likely the best way to get A's apprentice out of the contention. But it is not easily taken in the situation as described in the OP.
By HoH:TL p.52 Apprentices, these are "taken by the first magus who makes a formal and witnessed offer to teach them the Hermetic Arts." To make the apprentice in question his, B would need to be able to claim, that A traded (Apprentices p.58ff Legal Issues) the apprentice to him - which did not happen and would hence require a lot of risky fakery from B's part.
C's rights to the apprentice are wholly based on his rights to seize the property of his opponent during the lunar month of Wizard's War - so on the precise time of the occurrence on the player covenant's doorstep. The player characters should be able to determine the ending of a lunar month with means from their covenant.
Perhaps I should stress, that by HoH:TL p.52f Apprentices an apprentice does not become a magus by just holding on to the coat tails of a maga passing by and reciting the Oath of Hermes to her. The apprentice needs to have passed the gauntlet set by his master to the master's content, and afterwards join "the Order in a formal ceremony", which involves swearing the Oath. Only accomplished non-hermetic wizards join the Order without the gauntlet, swearing "the Oath (having been invited)."
Once the situation at the player covenant's doorstep is settled and neither B nor C can claim the apprentice, there might be a situation for a Quaesitor to step in and set a gauntlet (by ArM5 p.107 The End of Apprenticeship) for the apprentice, thereby finally giving him an opportunity to join the Order.
Sure. But its also clear that the Gauntlet is whatever the master says it is, and that the order doesn't second guess unless there's a suspicion they're making it too difficult to extort labour. The various Houses have their own traditions, but if someone wants to say "your Gauntlet is to stand there for a minute while I try and remember the Oath", its up to them. It may result in negative reputations, but its perfectly legal. And its exactly the sort of low standards other magi expect from Ex Miscs.