Impact In Ice Sheet
I've heard basically the same theory in the documentary "Mammoth Mystery". The difference is that the lack of a crater was due to impacting the ice sheet. The storm of smaller bodies seems to be a plausible alternative. I think the impact theory is far better than the others:
Disease: All documented plagues that I know of leave survivors.
Humans: I believe humans coexisted far longer in Asia and Europe.
Climate: There were many previous warming and chilling periods.
Regards,
Mark