Rabid lover of the totally separate universe's idea, just don't think that the universe shown after the Narada's arrival could have made THAT many changes in a mere quarter century solely because one ship (Kirk's dad's) was destroyed. So probabaly the universe that Nero emerged into was already substancially different place to start with, simular in some ways yes, but still a separate alternate parallel universe.
That's another theory I completely agree with. I think that OldSpock and Nero emerged in an alternate universe that was similar, but still substantially different than the one they came from. Again, despite what the writers would have us believe. That would explain a great many things that simply don't add up in the movie.