Actually, the multiple-Astlans issue is discussed in another thread.
When people travel to the Abyss, it collapses their alternate time-stream selves; bodily transfer i.e. a portal opening to the singular Abyss collapses the time streams of the world to which the portal is opened.
Thus, the more frequent a plane (and I mean an entire plane e.g. localverse) interacts with the Abyss, the fewer alternate time streams there are.
As a consequences of Astlan and the localverse seeming to be the favorite “vacation spot” for arch demons, Astlan and it’s localverse don’t actually have a lot of time-streams.
It’s Schrodinger’s cat, the more the singular Abyss “measures” the state of a particular universe, the more collapsed its wave function is. I.e. you only have one state.
Conversely this can also, in the case where it doesn’t happen that often, serve as a bifurcation point to create permanent alternate universes. E.g. permanent alternate universe because more than one of the timestreams is strong enough on their own to survive as a full universe.