How about the Mutantum magic of Mercere? Should that not receive some emphasis?
I suppose the other house specialities are essentially non-Hermetic and only partially integrated at best, so it makes sense to leave them out.
I wonder about Mercurian magic though. Is that too non-Hermetic to be valued?
You said, I think, that he considers Bonisagus' theory complete, but presumeably he would not want to give up the unified art of Vim and return to realm specific arts. Are you sure that no further improvements and simplifications should be contributed?
I believe some of these could be handled with Spell Mastery.
As for Vim, good one.
Maaaaaybe he could take some Vim deficiency or flaw, and state that "the 4 realm division was actually better and more effective, you've ruined it all and made bonisagus magic useless!", blaming on others his own deficiencies.
You are the SG making an NPC. Just have him with the virtues and flaws you want and they don't need to add up.
Make the major hermetic virtue you want
Heir of Bonisagus
You can trace your bloodline directly back to Bonisagus and have a strong connection to hermetic theory. Through a lot of study, you've solved the issue Bonisagus had with some of the more esoteric founders traditions.
You have a heart beast, verditis magic (without the need to uses items), enigmatic wisdom and faerie magic.
That's just one example of what you can do. I think you want to more focused on the purity of Bonisagus vision so my suggested virtue is completely off beam, but as the SG, it doesn't need to be fair or make sense.
Yes, of course, it does not have to comply with the rules of player characters, but it has obey the laws of the narrative, even if those are the laws of faerie which make their own sense and no sense as we know it.
That's pretty much the vibe I was going for. A story guide can cheat as much as they want to achieve an outcome, within reason. They don't need to follow the rules for one simple reason.
No matter how much of an OP munchkin a character is in an game, the SG can casually kill him, using the rules. This guy hates you so much, he found an arcane connection, has spent 12 seasons making the perfect items to combat you and you are dead. A more detailed "Rocks fall, you die."
The SGs job is to make a story. If they spend time trying to make the numbers work, especially during a session, that's time that can be spent more wisely. Hand wave the numbers around NPCs unless it is essential.