In most cases where eco-people (some scientists, some not) have predicted irrevocable damage, recovery has been quicker and better than they believed.

Ecological doomsday harbingers have a very poor track record.

I'm not sure it would be as easy to survive on one's own as they describe but they also do not say much about organized survival preparations. With a year or so lead time a lot could be done and I doubt that the lame attempt to divert with a nuke would be attempted. Anything that we did attempt would have a high probability of success or we would spend the resources on shelters.

Planet recovery would probably just be a matter of a couple of years but if they are correct that surface temps would be over 250 degrees, survival for that couple of years might require an organized effort and not just holing up in a cave.

It would be one helluva setback but civilization would survive, I believe.