From the episodes and movies, I think we can extrapolate some general rules of thumb, allowing for a few variations within the specifics.
Again: I didn't just "think" about this, I actually did it, in great detail, as research for
Watching the Clock. And there are a lot more "variations within the specifics" than you assume, and a lot of contradictions and completely nonsensical premises that I had to bend over backward to concoct a remotely credible-sounding scientific explanation for. This model of retroactive alteration is elementary compared to some of the convolutions I had to come up with. And
Idran's point about how the absence of known time travels affecting the past should result in the past happening differently is very logical, and canonically supported by "Yesteryear," where Spock's failure to time-travel in the present has a retroactive effect on his past.
(I haven't done the level of research you have, obviously, but I have read some things, and to be honest, the time travel stories never seemed quite as inconsistent as you've stated.)
I will never understand people who think their lack of knowledge of a subject makes them more qualified to judge it than people who have far more knowledge of the subject. Ignorance is not an advantage.
Then what's the point of making changes in the first place?
What a bizarre question. What's the point of the Mirror Universe, or "Yesterday's Enterprise," or reboots of James Bond or Batman or Godzilla? The point is to explore new variations on an idea. That's how art frequently works -- you take something familiar and change
some things about it while keeping the core of it recognizable. Like a composer writing a symphony based on a well-known folk melody, or a rock band doing a cover of an old song in a modern style. It's the contrast of familiar elements with a new context that makes it interesting.
In this case, the retroactive-change model serves two very clear and logical creative purposes. One is to provide an explanation for discrepancies in the past movies -- which, yes, I'm sure I've already acknowledged multiple times in your hearing as things that
can be interpreted as artistic license, but it's nice to have an alternative interpretation for those who find that interpretation unsatisfying. The other reason is to open the door for future filmmakers to introduce greater divergences if it's creatively useful to do so -- the benefits for which should be obvious.
Wasn't he always a good bit older than Kirk?
Jeffrey Hunter was about 38 when he did "The Cage," set in 2254. Bruce Greenwood was maybe 52 when he made ST'09, the bulk of which was set in 2258. Do the math.
I think that's nitpicking to the extreme, personally.
Says the guy who refuses to accept a timeline model outside his rigid assumptions.