Another issue I don't think I've seen anyone brought up: if the troll was identified as the Defense Professor, and Quirrel is then just an ordinary adult wizard as far as the Hogwarts wards are concerned, how was it possible for him to frame Hermione for Draco? Wouldn't the wards have screamed at the Memory Charm? It's pretty clearly implied that it had to be a professor, in some way, who cast the key spells:

"And how would Lucius even Memory-Charm Hermione in the first place, without setting off the wards? He's not a Professor - oh, right, you think it's Professor Snape."

"Wrong," said the Defense Professor. "Lucius Malfoy would trust no servant with that mission. But suppose some Hogwarts Professor, intelligent enough to cast a well-formed Memory Charm but of no great fighting ability, is visiting Hogsmeade. From a dark alley the black-clad form of Malfoy steps forth - he would go in person, for this - and speaks to her a single word."

"Imperio."

"Legilimens, rather," said Professor Quirrell. "I do not know if the Hogwarts wards would trigger for a returning Professor under the Imperius Curse. And if I do not know, Malfoy probably does not know either. But Malfoy is a perfect Occlumens at least; he might be able to use Legilimency. And for the target...perhaps Aurora Sinistra; none would question the Astronomy Professor moving about at night."

Probably this could still be rescued, but the troll theory, while bizarre, also means that the earlier frame becomes more complicated than before.