My take on AU Universes are they co exsist along side our universes and but do not directly interfere with each other unless you can hop inbetween them as we have seen in Mirror episodes.
A tidbit to consider:
We have never seen anyone travel through time by directly going through a black hole in Trek
before. As such, the potential consequences of time travel via such a method are completely unestablished. What if the consequence of time travel directly via a black hole -- not by coming close to one, as with TOS, but actually through
it -- is to cause the era into which one emerges to then branch off into an alternate universe where it had previously been one?
This would be completely consistent with prior Trek
installments' depictions of time travel even if
we reject the idea of alternate timelines branching off from one-another normally, because this is an unprecedented time travel method.
So you can only do time travel in the universe your in
Canonically erroneous. "In A Mirror, Darkly" firmly established that the USS Defiant
both travelled from the regular universe and
travelled back through time from the regular universe's 2268 to the Mirror Universe's 2155.