So yeah, about the fake Merlin thing, that reminded me of something that's been tugging on my mind for some time. Specifically, one of Britannia's mysteries that Varghese mentioned to Percy all the way back in the first chapter:
^That's from the official TL, but I've also seen fan translations that referred to that mage as one who "closed off their heart" (in fact, a translation with that particular phrasing can be found in the very first page of this thread). Anyway, my point is.... could this mage in question be Merlin? Like maybe she just plain gave up on Arthur after he changed, and in her despair, decided to seclude herself in that tower? It's certainly looking more likely now that we know the Merlin we've been seeing up to this point was a mere fabrication....