...2. Trek has used both "our timeline was changed" and "we created a new timeline" versions of time travel.
Trek has never said that a new timeline left the original intact.
It's always been said the one original timeline was changed.
Please give me an example of even one episode where they felt a new offshoot timeline resulted from the past being changed.
If that'd ever been said, the characters would have been stuck in the offshoot, with no way of EVER getting home, since changing the past a second time would only have created a THIRD timeline, not returned them to the original.
I just cited one, gastroff
-- "All Good Things." And then of course there's "Parallels," which establishes that alternate timelines branch off from the regular one all the time, as does "Relativity" -- to the point where they need to "temporally re-integrate" two versions of Captain Braxton. Then there was "The City on the Edge of Forever," wherein Kirk and Spock managed to change their own timeline, but wherein the prior timeline seems to have somehow continued to exist in some way, given as how Spock was able to gain information about the prior flow of history to appear on his tricorder from out of the ether. Then there were the Sphere Builders, who possessed the technology needed to observe and examine alternate timelines in order to determine how to manipulate the main one, which means that alternate timelines exist and branch off from the main one.
In essence: Trek has had it both ways, and there's no reason we can't interpret this new movie to have it both ways.