What used to be his home covenant. Is it still active?
Does it appear that the cat is possessing, manipulating, or controlling the Magus? Or that they've somehow switched minds?
And is he bigger than a breadbox? 8)
He waves off certamen with younglings like you 
(Incidentally, though I wanted to make this location independent, the saga is a Rhine one, so only an Archmagus can challenge another to certamen).
Two apprentices. One disappearedd mysteriously after gauntlet. One was killed during the Schism war.
Hmm, it's hard to get precise, reliable information on the sigil of a wizard who's disappeared over a century ago, but if you did, you'd find that yes, it seems to be consistent.
He's not supposed to provide any season of service. First of all, the covenant requires none from its members.
Even if it did, it would probably not require any from this senile, honored elder who may well be working 24/7 to save the Order.
What do you mean by "waves off"?
Actually, I believe that in the Rhine the rule is that anyone can challenge an archmagus, but that the archmagus wins by default against a lower ranked magus. An archmagus can still choose to actually fight certamen against younger ranked magi.
Also, in your saga when (chronologically) was this certamen practice introduced into the Peripheral Code?
Good question.
In his youth he was considered an rising star of illusion magic (Imaginem and Mentem). You may or may not find out that this stems from his early apprenticeship with a Jerbiton magus before being "requisitioned" by his Bonisagus parens.
Later on, he became obsessed with exotic magic, particularly that stemming from other supernatural realms - mainly faerie, but he conducted some research into the divine (he was one of those who brokered the truce with the kabbalists of Barcelona) and some say even into the infernal (from a purely academic standpoint!).
He does not appear particularly discomfited by it.
He is particularly saddened by intolerance against exotic magics (which he believes a great source of knowledge for the Order), and more in general, by any potential loss of knowledge. Fairly typical of many members of his house.
Doesn't the covenant consist of a number of young player character magi? Assume that the player characters decide to re-write their covenant charter to require covenant service. Seems perfectly sensible, especially if you have an elder magi available that you can thus "force" to do something useful for you. In this case, what does he propose to do? Or does he try to block the younger magi from doing this?
Someone said that all cats manipulate their masters
More seriously, at least superficially, it does not appear to be the case.
Of course, if an intelligent, magically puissant cat manipulated an old, senile, brain-addled person - how would you tell?
You have to be more specific about how you would challenge him to certamen. He is a jovial, honored, senile elder. Challenging him to certamen for anything other than sport would seem extremely boorish - a little bit like challenging him to a wrestling match. You could politely ask him to compete with you for sport ("I wish I could have the honor of honing my skills against such a powerful adversary") in which case he declines with good grace ("HOHOHO ... young blood, always looking for a fight eh? Ah, I recall when I was young blah blah blah ... ")
This is indeed the case. I was oversimplifying. Still, effectively you cannot force him into certamen.
Hmm ... I guess pretty early, probably well before the end of the tenth century.
Yes.
Well, for some reason it appears that the players really would not want to play such a covenant. But even if they did ...
... they'd really never think of press ganging this honored elder into service. It would be a little like hosting your friend's grandmother at your house, and insisting that she take her share of the daily chores of washing, scrubbing etc. It's simply not done, and would make the youngsters despised by the rest of the tribunal.
Well, it seems to me likely to be tied directly or indirectly to his interests - illusion, Realms, exotic magic. But the process to discover that would be lengthy, and more important, the motivation to research that is almost entirely lacking.
So whether or not your Players can unravel this mystery quickly is secondary to the question - Why would they try?
You say there's a mystery behind him - if the Players/Characters don't know there's a mystery, if they can't be made to care where this duffer has been hiding, then... they don't care.
PC's need a reason - and usually more than just an unanswered question - to pursue something like this. If they don't get a hint that this mystery needs to be solved, and fast, and that hint isn't in the form of a big carrot, big stick, or both - they have better things to do with their time than sift through the dustbin of this mage's personal trivia.
I completely agree. I was wondering, however, if there was some mechanical aspect that I had not thought about that would inadvertently spoil the plot, and if the way I presented the information gave away more than it should.
I don't know. How close to the mark have we come so far?
From what I've seen, I'd connect the following:
o His age. (Not because of how old he is, but where it puts him in the Hermetic Timeline when he was active)
o His interests - Realms, Illusion, and Exotic Magics.
o A philosophically/politically motivated connection w/ House Diedne.
o An apprentice who went mysteriously missing after Gauntlet.
o ... and the statement of "avoiding a doom"
At the risk of jumping to conclusions, I'd worry that some Element of House D was just over the horizon. And I'd worry about that apprentice, too.
The fact that he was a Jerb apprentice grabbed by a Bonisagus tells me he'd still have an interest in "relations". Hence his empathy with the position of House D, even if he never touched Diablerie (which may or may not have been a just accusation of House D to begin with!)
(But you have to remember that in this type of thought process, synergy is extremely valuable - being able to play off others ideas is a huge advantage, one a handful of players in a closed environment won't have.)
Out of interest how does the covenant acquire resources for common projects (like casting the Aegis of the Hearth) if no-one does any covenant service?
Of course, it's your saga, but this seems like an odd attitude for the covenant (and Tribunal) to take. Someone claiming to be the oldest magus in the Tribunal has turned up unexpectedly. Everyone in the Tribunal would be interested in a) taking advantage of whatever magical secrets they can prise from him, and b) confirming that he really is who he claims that he is.
The magus who is most likely to be concerned about him is the second oldest magus in the Tribunal (i.e. all the other archmagi). If the other archmagi of the Tribunal are not snooping around, and not setting minions (like the young magi of this covenant) to snoop around on their behalf, that would itself seem very suspicious indeed of something weird being up.
What is the challenge that this archmagus sets for masters wanting to be recognized as archmagi?