[Update: and now there's a fifth discussion thread, which you should probably use in preference to this one. Later update: and a sixth -- in the discussion section, which is where these threads are living for now on. Also: tag for HP threads in the main section, and tag for HP threads in the discussion section.]
The third discussion thread is above 500 comments now, just like the others, so it's time for a new one. Predecessors: one, two, three. For anyone who's been on Mars and doesn't know what this is about: it's Eliezer's remarkable Harry Potter fanfic.
Spoiler warning and helpful suggestion (copied from those in the earlier threads):
Spoiler Warning: this thread contains unrot13'd spoilers for Harry Potter and the Methods of Rationality up to the current chapter and for the original Harry Potter series. Please continue to use rot13 for spoilers to other works of fiction, or if you have insider knowledge of future chapters of Harry Potter and the Methods of Rationality.
A suggestion: mention at the top of your comment which chapter you're commenting on, or what chapter you're up to, so that people can understand the context of your comment even after more chapters have been posted. This can also help people avoid reading spoilers for a new chapter before they realize that there is a new chapter.
Yeah, I think the naive reading is that the narration is from Harry's POV, and Quirrell Polyjuiced into the man (as planned), and Quirrell is such a badass that he can do whatever magic he wants while polyjuiced, which is unusual enough for Bahry not to expect it.
And Quirrell claiming there's no plausible reason to think he's possessed by Voldemort is just him thumbing his nose at the reader (aside from the usual misdirection).
If it turns out to be just "Quirrell is such a badass" then I'll be very disappointed.
My reason for choosing A over C wrt: possessing Quirrell is not that Quirrell lying is implausible (that much is entirely likely) but that it raises the question of why, if there is a plausible reason, and Harry was invited to think of one, he didn't come up with any.
That said, we're only getting cherry-picked fragments of Harry's thinking, and he's being manipulated anyway. So maybe Harry just isn't thinking straight.
Still, until I see a plausible reason to believe it, I don't.