The timeline/universe thing has always confused me. Because there are a number of theories out there, and it seems Star Trek has explored many of them. But in doing so, they seem to say that all are of those possibilities exist. So, you can go into your past and change your own timeline (in these cases, our characters try not to do anything major in the past to stir up their present, and if they do they have to fix it/make it right before going back to their present). But you can also go into the past in an alternate timeline. Or the present in an alternate universe.
I think if I were Spock (Prime), I would try to figure out a way to get to his original timeline and time period if that's possible, and then see if what happen changed his future or in fact created a completely new timeline which he is now stuck in. There's really no way to find out for sure unless he attempts to go back. Unless I'm completely wrong, which is very possible because as I said all of the timeline stuff confuses me. And I've only seen the new movies once each, so my memory of the definitive talk about their timeline in the films themselves are a blur now. If he is indeed in an alternate timeline and he knows that for sure, then he has nothing to lose by trying to get back to his own timeline because there is an infinite number of alternate timelines and he's just in one of them. And the one he's in sucks now with Vulcan destroyed and everything, so all the more reason to attempt to get back to his own timeline. Seems logical to me.