As an anthropologist I think the extinction of humanity isn't as big of a threat as we think. We've been around as a species for about a quarter million years and survived everything up until now. We are a pretty resourceful bunch! Life may look different in time but I think we'll be okay

Theoretically I think it's possible (even though this is not my area of expertise), but the ethics of this are risky. Ice from other planets could hold contagions which could cause unknown damage to our ecosystem. It would be an expensive way to bring about another mass extinction event