And that prevents him from choosing to go back intentionally to fix all this....how?
It's simple. Slingshot around the sun to go back to a point before the timeline diverged and prevent the destruction of the Kelvin, presuming that's the focal point in the timeline (another argument), then go back to the point in the timeline before this whole thing started, apparently the destruction of Romulus. Prevent that, or at least mitigate the results somehow, to the point where Nero doesn't go on a time-spanning rampage, and bingo, timeline restored.
But then, JJ wouldn't have his own Star Trek universe to do with what he wants.
All Spock has is the Jellyfish. How do you know he can even defeat Nero head to head? How do you know that the timeline might not get screwed up even more?
Anyway, your time travel argument is moot because that idea can be applied to ANY instance in Trek when a disaster happens. Why not just go back in time and fix it?
The fact that you are using that point to try and rip this film, but choose to ignore all the other times in Trek that the time travel plot device can be used completely destroys your argument.