Re: Timeline branching going forward *spoilers*
Howso? Spock and Nero travel between them.
They do not. Nero creates it, Spock follows him through the same portal (black hole), and it's a one way trip.
I don't recall anyone saying at any point that Spock and Nero couldn't possibly return to their timeline of origination. It would likely be extremely difficult of course.
Heck, for all we know the Red Matter hole created at the end of the film leads back to the Primeline.
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