A comet getting close to earth doesn't butterfly away the Manchus.
I think it would if it was that far back. The Manchu weren't even called the Manchu until the early 17th century, before that they were the Jurchen. And considering just how volatile Asian steppe history is, and how many steppe peoples arose to do their dance and then get wiped out completely (Pechenegs, Cumans, Dzungars, etc), I think that the rise of the Manchu is something that would be very vulnerable to butterflies.
But I'm still curious as to how things are going.