All you are telling me is that most time travel in Star Trek has been innacurate with regards to current prevailing theory, and somewhat inconsistent anyway.
All you're doing here is repeating O&K's claims about "current prevailing theory." There is no
"current prevailing theory" about how time travel would work (although there are plenty of speculations), so realistically they were misrepresenting what the MWI means in order to rationalize their story choices about how to execute a reboot.
UHURA: An Alternate Reality.
This bit of expository dialogue was, obviously, O&K's attempt to get the point across to the slower members of the audience, slightly less blunt than having the writers themselves walk in front of the cameras and say "this is what we mean." However, at that point in the story it can realistically be nothing more than speculation on Spock and Uhura's part ("alternate to what?" is the obvious question from their POV), and moreover it provides no evidence one way or the other as to the status of the "prime" universe.