Okay, after this, I'm done for the night. I need to go to bed.
Unless they haven't figured out how to feasibly travel between realities in the 29th Century. In that case, once the Relativity's timeline is altered, they can't go back to their unaltered reality. They have no choice but to correct a tampered timeline because it's the only one they have access to.
but in your explanation, every altering is just another alternate reality, so the fact that they've been able to alter things before, means they are able to travel between alternate realities. plus their goal is to prevent the timeline from being altered so knowing where the incursion occured, their goal will be to prevent the incident and restore the timeline. though by restore, it would be more like preventing the incident so that the incursion never happened therefore negating that alternate reality as it would have never happened. so, still following your alternate reality train of thought, if they went back to a point in time before the disruption occured, they wouldn't technically be on a different reality as that alternate has not been created yet, hence they would have no problem getting to it, if they indeed had a problem traveling to alternate realities.
I think the new Trek timeline no longer has anything in common with the old Trek timeline much earlier than 2233. Let's say these two timelines have everything in common up until the 1990s, as an example, and then they branch off from each other. In the Relativity's reality there were Eugenics Wars from 1992-1996. In the New Trek reality, everything up through at least 2009 occurs as we know it in real life. So the New Trek reality is a parallel universe like the Mirror Universe. It's entirely possible that the Mirror Universe also has a common past with the Prime Universe but naturally diverged, just to make up a number, four million years ago.
Maybe the Relativity doesn't need to be concerned about what Nero does in a parallel universe that was never intended to lead up their reality in the first place. In other words, 2233 in
Star Trek was never the 2233 in the Relativity's universe and it would be impossible to reconcile the two universes. It's not just one or two things different about New Universe or one or two points that can be blamed for all the differences. There were differences that unfolded for several reasons going back several hundred years.
The Relativity might examine realities on a case by case basis. The degree of divergence might play a major factor. How fundamentally different is a reality from theirs? And are they dealing with an Altered Timeline or a Different Timeline?
An Altered Timeline would be something like "Yesterday's Enterprise" where one change due to a temporal anomoly alters everything else. A Different Timeline is a timeline where there are several differences which had nothing to do with any anomolies and have to do with how that timeline naturally developed. So Nero disrupted what was already a fundamentally differnet universe to begin with.
Because of the soft science with the black hole, Nero and Spock don't have to travel back in a straight line. They can go sideways in time instead of just back. They're going into a time/reality that Relativity might not even be aware of.
