If Spock goes back to a time before 2233, via ANY means, he will then create a divergent timeline in which he will exist.
It would not affect the timeline created in 2233.
Since either the Prime Reality, nor the Alternate Reality established Stardate 2233.04 would change, the exercise would be pointless from Spock's perspective.
Once again, I don't follow your reasoning here.
Your premise seems to be that the very act of time travel into the past immediately and irrevocably creates an alternate timeline, and therefore that any attempt to undo a "previous" time alteration is inherently futile.
If this were true, however, it would invalidate essentially every time-travel story we've ever seen on Star Trek.
You may argue, of course, that O&K are "correct" in their assertion that the MWI governs everything and thus that all those past adventures involved our heroes jumping around from one timeline to another, without ever actually correcting anything or getting back "home." I think that's an unnecessary idea in scientific terms and an idiotic one in narrative terms, but regardless... even if it were true, it'd be moot here.
Why? Because we're talking about
what Spock's motivated to do, and from his point of view, all those past time-correcting jaunts
worked. Spock, based on subjective experience, has no reason to share the writers' perspective and every reason to think the exact opposite.
Let me return to my "fork in the road metaphor." There's a road: that's your "prime" timeline. Someone comes along and starts digging up and paving a fork that moves off from it in a different direction: that's your "alternate" timeline. The original road still exists where it was (per the terms O&K themselves laid out!), but once you've set off along the new fork, you can no longer get to it. However, you can travel back along the road to a point
before the fork -- where there's still just one road, a common past -- and do something to stop the guy who started the fork. (Disable his earth-moving equipment, whatever. The metaphor gets strained.) Taking this trip back into the common past
does not automatically create a new fork on which you're once again trapped. You seem to be assuming it does, but that's an unwarranted assumption.
This is perfectly legitimate plan of action, based on all available prior evidence about temporal mechanics in the Trek universe, and all of Spock's knowledge of same. (Consider "Trials and Tribble-ations," to add another example to the time-travel episodes I've already mentioned. It worked exactly like that. Arne Darvin went back to change things, the DS9 crew also went back and worked to stop him, and the timeline remained unaltered.)
Moreover, Spock could hardly ask for a more advantageous position. He knows the exact time and place of Nero's arrival. He knows Nero's capabilities, and his motivations (such as they are), and his weaknesses. He has the full resources of 23rd-century Starfleet at his disposal, as well as all the additional knowledge he possesses from an additional 130 years of experience with temporal mechanics. It would make
all the sense in the world for him to try to go back and prevent the timeline alteration that led to the deaths of billions and the destruction of one of the Federation's founding planets (and also, by the by, ruined the childhood of his best friend).
So why doesn't he at least attempt it?
This is not a trivial question.