My thoughts on the matter:
Even if a blackhole was a door way to another universe, or a seperate "baby" universe itself, it would be impossible to travel "through" the blackhole as the vast tidal forces are sufficent to destroy any structured matter that gets close to the event horizon.
On communication from "the other side" as nothing can escape the blackhole, no particles or radiation can be passed from one "side" to the other. However there maybe workarounds for this using quantum entaglement between two particles to transmit information.
The accretion disc is gas that is attracted by the blackhole and "orbits" the hole. accretion discs form in binary star systems where one body has colapsed to form a singularity and "steals" matter from it's partner. As binary stars are quite common, lots of black holes have accretion disc. As the gas rotates the hole friction causes vast temperatures which is why we can see the disc at all. The gas orbiting the blackhole gets pulled in when it's orbit becomes unstable, maybe due to it's partner stars influence. As for the accretion disc jets we just dont know why they form. Probably some magnetic function of the hole, but they certanly DONT come from the hole itself.
On the size and shape of the universe, my own opinions give 2 answers.
1) the universe is infinite and has no end. Personally i dislike this theory, especially when we have a date for the beginning, at 13.7bn years ago, however the universe is over 13.7bn ly across as far as we can tell. So light starting out at one end still hasn't got to the other end. Which to me means that the universe IS infinite, at least for all practical purposes.
2) the universe isn't infinite. Which dictates that it has some boundry or edge. Again i dislike this theory as it smacks of sci-fi from the 40's when spaceships would come against some invisible wall and bump against it like a glass bottle. Inevitably this begs the question: what's on the other side of the wall? Most peeople wont be happy with the answer "nothing".
3) there is a 3rd answer which is a mixture of the other 2, where the universe is finite, but has no boundry or edge. Consider a sphere to a flatlander, there is no edge or boundry but you can walk forever, and if you walk far enough you can start back where you began. This seems the most likely explaination to my feeble mind. The universe is a sphere only in higher dimentions. A 4-sphere perhaps. or a 6-sphere. or a 27-sphere...
I recently read an article in the New Scientest on Burkhard Heim. Very interesting and reccomended reading for popular science fans like myself. Online article here
It's questions like these that keep you up at night hehe