Another possibility is that there's a separate curse. Broken curse completely prevents a Riddle from trying to kill a Riddle, second curse imposes consequences. Maybe if one Riddle kills another Riddle, all the Riddles die -- that would explain why Quirrellmort hasn't killed Harrymort. All he says in Parseltongue is that Q may kill H any time Q wishes; there's nothing that says that Q killing H would have consequences bad enough that Q won't.
I don't know if there's any evidence for this, but it is a possibility that explains why Q hasn't killed H and doesn't require lying in Parseltongue. (If there's a second curse, could that be behind the resonance? Do we know what causes the resonance?)
Another possibility that fits is that prophecies are not to be messed with in that manner.
It could indeed be that he isn't actually lying in parseltongue, as you suggest. I think I've come out too strong on that point. But the threat to kill Harry is a bluff. Voldemort has some really compelling reason to not kill Harry.
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 112.
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.)
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: