Given the highly unstable nature of the results of the Genesis Device, I doubt Spock will use that - he's already said he's found a colony world.
As for the temporal issues, this is nicely set up.
Either:
1)This really is an completely alternate timeline. Many-Worlds-style, the old timeline still exists, and if they want to play in it, they can. This is the most likely, as the current situation is highly paradoxical. With Vulcan destroyed, the Vulcan Science Academy cannot build the ship that tries to save Romulus. So, the reason and method for Nero's time travel does not exist in the new continuity.
Or:
2)Spock writes down everything he knows about the original continuity, and the deviation from original timeline (which is considerable). Assume that while there's been changes in Federation development, the stars themselves don't give a hoot - Romulus will still be destroyed in this timeline. In 120-someodd years, they either stop that from happening, or evacuate Romulus in time, so that Nero has no reason to go back in time and cause the branching. This results in a temporary 120+ year loop of divergent continuity they can play in until they decide (if ever) to restore the original.
As for temporal inertia - I find it... extremely hard to believe that Kirk/Spock/Enterprise has more inertia than several
billions of people lost on Vulcan. Nero's actions can destroy a world, but the crew has too much inertia to disrupt? I don't buy that.
And let us not get into the Temporal Prime Directive. Things get messy