A major flaw, such as Waster of Vis, gives you three points with which to buy virtues.
It's a distinction with a reason-- Mercere the Founder was the original Mercurian. It makes sense that his bloodline would make better Mercurians than others taught the virtue.
The original Mercurian? First as far as being inducted into the Order, I'll grant you. His gift was already developed at that time, he had a master, at some point, could that master not have had two students? In any event, Mercurian is Mercurian, and the House doesn't make a difference, to me. That's what the House Virtues and house-only offerings are for. Only Mercere can be Mutantes and take those virtues associated with that Lineage. Could a lot of house Mercere be Mercurian? Sure. But I'm not sure I see a reason why the Flambeau Mercurians have to be different from the Mercere Mercurians. Differentiation can be achieved through other House only virtues available to House Mercere.
For the apprentice thingie, my take is a little different.
I hate the idea that virtues/flaws are taught, or at least easily taught. Especially with your fosterage idea, it stands to reason that the next generation of hermetics might be well better, with multiple major virtues.
My take is more like this:
This, okay, although I'd add a -5 when not casting spontaneous magic ceremoniously.
With all due respect, I hate this.
IMO, a given virtue should work the same way for every player, or it is quite unfair. If it doesn't, if should cost more/less.
It may sound surprising, but I'd feel waaaaaay more comfortable with you restricting Mercurian Magic to Mercere magi only (save a Dark Secret).
Not found.
So:
Black Stone of the Berserker: a small, black stone, that casts at Touch range the spell Endurance of the Berserker for Diameter duration. Final level 20.
He'll hand these to the covenant, as a thanks for his welcome.
Laetitia would like to claim Silviatos' lab.
Well, as I said, I thought about it. The -5 is problematic on many levels. If you take it after dividing that means Te+Fo+Sta+Aura has to be 30 or greater to get off the simplest of spells, that combined CS of 30 gets dropped to 6 and then to 1. If you do it before dividing, it's applied inversely, meaning fatiguing yourself, with respect to the penalty is actually worse than not fatiguing yourself[1]. So the -5 has lots of problems. Also, it would remain the only Major Hermetic Virtue with a mechanical penalty, when the benefits of the Virtue itself are still suspect. Half cost for rituals sounds great, but how many rituals do they cast? Free Wizard's Communion of highest level ritual you know, but you probably can't cast it, because your Arts are insufficient to cast the spell. In the end adding +5 if they do ceremonial spells makes more sense, it's an incentive to take the virtue, and to be focused on casting ceremonial magic/ritual/slow magic. Rather than a stick that says you're bad at this because you can do that other thing really well. IMO, that should be reflected in flaws to balance out virtues, not within the virtues as it stands.
[1] If the -5 is applied to the CS, for non fatiguing spells it becomes a -1 (5/5), but for fatiguing spells it actually increases the penalty (5/2) even though the total amount might be higher, the -5 penalty is worse when fatiguing ones self. It's easily swapped around, but that's a headache waiting to happen, this and other reasons indicated above just make it a bad idea.
With Fieltarn and Jormungand being absent those are other locations to pick. They are of sufficient size. I'm retconning Jormungand's away, but not the total effects of it.
Scratch this taken to pm...
With Ulrich being removed from Mons Electi and spending the summer and fall at Le Maison de Levrier, and presumably moving to Phoenix that winter, Silviatos's sanctum and lab is now vacant. Although Ulrich (or Tria) will likely be sending for his things before too long.
What things does he have? That boy's had far too much freedom.
Order of things is kind of messed up from a story POV, and requires some retcons of Laetitia takes Silviatos's lab. Silviatos's lab would be vacant when Isen arrives (and he would've been absent for over a year), it makes some sense that he takes it and kicks Ulrich out.
Laetitia arrives in late fall of 1221. Apollodorus blessed the idea of adding her, and she took part in the Aegis ritual for ME. I see I need to answer your question about a parma text for 1221: it's a L6Q15 Summa.
https://forum.atlas-games.com/t/seasonal-activities-thread/6263/33
Summer of 1222, according to that was Laetitia taking over someone's lab, Fieltarn's or Jormungand's: both are essentially empty +1 sized shells, their previous occupants having acted as if they were being foreclosed upon (taking all the good stuff out, and trashing it otherwise).
Not much, really. Just some clothes, bedding (which is the covenant's, anyway), tools for whittling, some stuff he had whittled over time. Probably everything would fit in a large gym bag or a small duffel bag.
Would he then have taken one of the other vacant labs, one of the cottages somewhere, or would someone have taken him in (possibly Apollodorus)?
Probably not. Remember, Apollodorus took Ulrich in, because he had warnings about Methuselah and what an awful master he was. What he did to poor Tektonius was inexcusable! So, he took Ulrich as a means of reigning in the worst attributes of Methuselah, not ever intending to take on an apprentice again.
He probably gets a room, somewhere, not too far from the library where the Magi can check in on him easily.
That's my story and I'm sticknig to it.
Jacques has claimed Jormungard's lab.
Or Fieltarns, or whatever. Bottom line, there's a lab out there, or space for one. Again, the principle of the saga is summertime and the living is easy. Kinda. 
Players should be advised that they really do have vast resources at their command. Not huge vis stockpiles, to be sure, but some very rich resources. Leveraging these resources is a key to solving many of the problems I place before the players.
I was more interested in the size +1 superior equipment than the location.
Dammit, my post got eaten. I'll post again in a bit.
Note that Viscaria also wanted to read the Rego book in Winter 1222. If the conflict is pressed, Viscaria will ask for an arcane connection to the book.
Note there is a much better Rego book that has become available.
I'm pretty sure I already planned for Korvin to be older than those three (all <5 years post-gauntlet).
If Korvin wants to start pulling on one of the many dangling plot threads in Phoenix, there may be Leadership opportunities for him, but so far it seems like he's not planning to be a nosy guest.
Yeah, but V doesn't read Greek, and there's no one to teach her, I thought. Besides, that's what the Perfectly Ordinary Mirror is for.
(Although it seems that Korvin is no longer opening her arts in 1222 -- I'm still trying to catch up on everything).
I didn't read any of page 35 of this thread. Let me know if any of it was important.