Which would allow Spock to return to the Prime timeline should he choose to.
In theory, but who says he has the means? His journey to the new timeline was not controlled; it was a consequence of the red matter, which a) was destroyed, b) is beyond the technology of the era he now inhabits, and c) would be too dangerous and unpredictable to take a chance on using anyway. And there is no known technology for the controlled transfer between timelines; the only one we've seen was the transporter gizmo used to cross to and from the Mirror Universe in DS9, and that was apparently only capable of transferring between those two timelines, as well as (again) not having been invented yet. (And yes, Spock Prime has considerable scientific expertise of his own, but there's only so much he could do without the necessary technological infrastructure. The Jellyfish ship was destroyed, so no 24th-century tech remains in the new timeline.)
That only leaves technology for travelling forward or backward in time, such as the slingshot effect or the Guardian of Forever. But would that avail him? That brings us to the next quote:
That raises an interesting question -- if someone were to travel back to just before the March 2233 split and live forward from that point on, which branch of the timeline would they experience? Or would they be split in two and exist in both?
If they didn't change anything, then yes, they would diverge along with everything else. Otherwise, they would create a third timeline that would branch off in a separate direction due to their presence, just as Nero caused a branching on his arrival. Since, objectively speaking, that timeline would actually have diverged before Nero's arrival, it would presumably be Nero-free and could be expected to develop similarly to the Prime timeline, except for whatever changes might result from one's own anachronistic presence.
But there's a more critical issue here that has nothing to do with temporal physics. Spock Prime wouldn't choose to return to his own history even if he could. He considers himself responsible for the destruction of Vulcan in the new timeline, because he failed to save Romulus and thus set off Nero. So he's dedicated himself to aiding the surviving Vulcan population, ensuring that the species recovers. He wouldn't abandon that responsibility merely in order to relocate himself to a more agreeable reality. That would be completely out of character for Spock.