Having seen the Terminator films and TV series, where those damn machines keep eventually being built even when the company behind them changes (Cyberdyne in T1 and 2, Cyber Systems in T3), when the origin of Skynet changes (a big mainframe, then a internet software program from the US Air Force, then a chess computer merging with something-or-other in the TV series) and the date of the nuclear apocalypse keeps changing but remains a constant event.... yeah, I can see the Enterprise crew uniting in a ton of different timelines.
I don't deny they could come together early "in a ton of different timelines". The question is: Could they come together in the actual timeline that branched off from the prime universe, given the disruption cased by Nero? Remember, this would be the same likelihood that would occur in a traditional time travel story if the prime timeline itself had be effected in the same way. So we are not talking about an infinite number of timelines to choose from.
Having TNG characters would certainly stretch it that much. The original cast showing up on the Enterprise isn't as bad as that, but the way everything lined up still strained probability.
Do you think TNG characters would stretch credibility purely because they are
from TNG, or because you don't think their existence can be explained well enough in the earlier time period? Ignoring Spock Prime of course.