So, what are the General spells in the core rules (I'm not checking all the books right now…)?
Ward Against the Beasts of Legend (ReAn)
Ward Against the Faeries of the Waters (ReAq)
Ward Against the Faeries of the Air (ReAu)
Sight of the True Form (InCo)
Ward Against Faeries of the Wood (ReHe)
Dispel the Phantom Image (PeIm)
Restore the Moved Image (ReIm)
Lay to Rest the Haunting Spirit (PeMe)
Ring of Warding Against Spirits (ReMe)
Ward Against Faeries of the Earth (ReTe)
Shell of False Determinations (CrVi)
Shell of Opaque Mysteries (CrVi)
The Invisible Eye Revealed (InVi)
All MuVi spells
All PeVi spells
All ReVi spells except for Gather the Essence of the Beast
Note that, while Wizard's Communion is a General Spell, Wizard's Vigil is not. It is listed as MuVi 40 on page 75 of Through the Aegis. So, as written, Adaptive Casting does not apply to Wizard's Vigil.
That does not strike me as a sensible distinction. The fact that it was, in the real-world history of the line, helpful to write Wizard's Communion as General, and Wizard's Vigil as a particular level, is not reflective of anything important in the fiction. So, I think "Is it described as General in the books?" is a non-starter. "Is it based on a General guideline?" is not much better, as @callen has pointed out that quite a few of the non-General guidelines can be written as General, with minimal effort — certainly no more special cases than Sight of the True Form. (With my philosopher's hat on, all spells can be rewritten with a General guideline, although the notation would be less than totally transparent.)
This is why I am inclined to rely on Similar Spells, which does reflect something significant in the fiction.
I am not inclined to merge Mastery Abilities, however. That is a significant power boost to Flawless Magic, even if you merge the XP rather than the levels, and Flawless Magic really does not need a significant power boost. (I think it probably is the "best" Major Hermetic Virtue, but it is not so good that you should take it regardless of character concept, so it is OK. It should not be made stronger by errata, though.)
(I also agree that Aegis of the Hearth is a special case, but it is a special case, and so we cannot generalise from that to any other General spells.)