Here is one for you. Did the mirror universe also get reset? Is it the nearest universe where it also saw Narada go back into time? Is it a universe somehow corrupted by a mad Q where it isn't just another timeline--but a true alternate universe modified by an evil entiry to where everyone seems to be in a bad mood--and not just because the way history turned out with conquests. We had a history like that but people are generally nice.
Now I'm starting to sound like Rom, in wondering what exactly is alternate.
Well first from our limited mortal view we can say that all parallel universes are alternate too. And the one or two mirror universes we know of are among them.
I would say that the mirror universe is only one of the neighboring universes.
I don't know where the inversion of the good people are bad and bad people are good came from. From on screen evidence it only shows that the humans are a bit more evil, and once they are broken for some decades they develop goodness.
As for the source of the mirror universe, no idea, my take is that the genetic defect of the Augments as observed by Arik Soong actually naturally developed in mirror humans in "prahistory".
As to the nuMirror universe, then, yes, sooner or later it will change. Now whether that will be when nuDefiant disappears or in the 22nd century, we shall never now, for I don't believe it will be ever addressed.