>> Except, do we need him alive when we destroy the horcruxes?
Tonks never knew anything about the horcruxes. She was killed before Harry beat Voldemort and Dumbledore told no one in the order, except Snape.
Also, wouldn't she remember grabbing/meeting the 1990 snitch when she saw the 1998 model?
Author's response
You're not the first person to make that first point. I have no commitment to canon, but I do have one to not accidentally confusing people, so I'll go stick some author's notes in there as to what my AU premises are. This was originally an error, assuming there's no canon support for my version, but I probably would have just made the change anyway. In any case telling me you're confused is actually helpful!
Second, the idea was that the effect was to move the subject sideways as well as back. It's not supposed to create stability and sanity, but rather to rip the old timeline out of existence. It's better than the ground-up fairies from your other comment, but it's still a little bit of hand-waving. :)