I think the obvious solution is basically this, with a Time-Turning involved. The troll could be real, or not (probably is). The hardest part about changing the past is faking the evidence including memories, but with a False-Memory Charm that becomes trivial. Memory charm Harry and possibly Dumbledore as well, depending on whether he objects "but I remember feeling a student die." They won't do it this way because it's too finger snap-ish and not dramatic enough, but if it's not at least addressed then I will allege a holding of the Idiot Ball.
Harry will need the help of Dumbledore or Quirrel to unlock the time-turner and cast the memory charms. Quirrel wouldn't help but I'd be interested to see his excuse; Dumbledore should be possible to convince but might not be. McGonagall or someone might be capable of it but wouldn't obliviate Albus without him asking for it. Unless this is what was foreshadowed with the question about her first loyalty?
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 88-89. The previous thread has passed 500 comments.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: