Except that Harry didn't have access to his time turner prior to witnessing Hermione's death. Once something has happened it cannot be re-written in the self consistent time travel of the HPMORverse.
Harry did ask Dumbledore about ways to create the illusion of death in order to get around this hurdle (essentially allowing the information to be the appearance of Hermione's death as opposed to her actual death), but the Headmaster said he had tried something like that with disastrous results previously.
I know of nothing in the known rules of time travel in HPMOR that wouldn't allow for Harry to plan to swap gems. In fact, he used basically this exact same trick for the Remembrall and Torture incidents.
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 94. The previous thread has passed 200 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, 19, 20, 21, 22.
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: