And there you go with the cave thing. I thought nobody would EVER find that contrived plot device to acceptable. I suppose what I don't understand is why the universe would care. This timeline is exactly how it is supposed to be. Many Worlds theory includes every possible timeline and every possible timeline includes timelines impacted by time travellers.
The way Roberto Orci rationalized it in interviews was in terms of probability. The idea is that certain outcomes are more probable than others, and that the majority of timelines will tend toward the more probable outcomes just as a matter of statistics -- which can give the appearance that the cosmos is somehow "pushing" things in that direction. So the screenwriters' thinking is that if something happens to push a timeline away from that high-probability track, there will be a tendency to correct back toward the high-probability track. It's a little bit fudged for story reasons, but it's not entirely devoid of merit as an idea.
The way I implicitly rationalized it in DTI: Watching the Clock
is that if there's an entanglement between two timelines -- if one is created due to intervention from another, but circumstances allow the timelines to coexist rather than collapsing together -- then that partial entanglement will sort of contaminate the altered timeline with aspects of the quantum phase state of the original, and that will affect quantum probabilities so that events in the altered timeline still happen similarly to those in the original. In my mind, this also explains why the Mirror Universe hews so close to the Prime timeline even after centuries apart; I figure the arrival of the Defiant
in the MU's 2150s created an entanglement between the timelines and created an influence that caused similar events to occur, the same people to be born and end up in the same places, etc., at least up to a point.
Anyway, it's not that
huge a coincidence that Spock Prime and young Kirk were on the same part of Delta Vega at the same time. Presumably they were both placed there so they'd be in proximity to the Starfleet station where Scott was: Nero put Spock Prime near enough to it that he could be rescued and have to live with the fate of Vulcan, and Spock sent Kirk's escape pod there so that he could be retrieved and protected until he could be brought back to Earth for court-martial. If the cave was the only shelter in the area, it's not so unlikely that they both would've ended up there.