(Can't find a good place to insert my entire current edifice of theory elsewhere, so I'll put it as a top level comment.)

Quirrell is Voldemort is Mr. Hat and Cloak. Quirrell's ultimate goal is driving Harry permanently into his Dark Side, so as to be another Voldemort, either to rule alongside the real Voldie or to be led by the real Voldie.

Quirrell's first attempt at driving Harry over to his Dark Side was with the Dementor in the Humanism sequence. He would have succeeded, had Hermione not been there to bring him out of it. So from Quirrell's point of view, Hermione is Harry's anchor to his good side.

So then it makes perfect sense for all the events of Self Actualization and this new sequence to be Quirrel/Voldie/H&C's handiwork. Quirrell helped SPHEW win that last battle in Chapter 74 so as to paint a larger target on Hermione. When Harry came to him with his plan for that battle, he just laughed, because Harry had unknowingly come to him with a brilliant plan to further Quirrell's own goals for Hermione's doom. He poisoned Hermione against Draco (which was difficult, because Quirrell is pure evil and Hermione is pure good and he can't understand her) so as to provoke House Malfoy further.

Now Hermione is in deep trouble, just as Quirrell intended. All that remains is for something unspeakably awful to happen to her soon, which, Quirrell believes, will help do to Harry what the Dementor almost did in January.