I don't think Harry really could preserve the Death Eaters' heads. Remember, he was out of magic, totally drained after the "Obliviate". And he had to take of Voldemort first, to not take any chance of letting him escape. So even while Harry is good at "doing the impossible", there is a limit to how much "impossible" he can do a limited time.
He might have a portable refrigerator set to 4 degrees C in that bag of tricks of his, but that's a little much forward planning even for him. And that still leaves the problem of safely cooling ten times thirty-six pounds of meat down to that temperature without damage.
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 117.
Plans for next chapter release:
There is 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.)