re: Do we need an actual impact crater for this to have happened?
While multiple aribursts would do quite well, see the links I posted to above.
In particular, see Ken Tankersley's site near Sandusky for burnt mega fauna bones.
For South America, see the Taima Taima link.
Those who were left did indeed repopulate the Americas.
I gave one big reason, indeed probably the main reason, for the lack of more evidence in my post.
A special on this one was shown on NOVA last week. I think many people just have trouble accepting large recent deadly impacts.
E.P. Grondine
Man and Impact in the Americas