[@Klomster]

I'd say 6 - 8 players would be nice, but a little more or less wouldn't be all that bad. (Preferably at least four players)

To return to the upper world (or one of the various pocket dimensions), you need to form a portal. However this may not be possible if you are extremely injured or are in the middle of combat as it takes a few moments to actually form a portal.

Deities do not have a "mortal form", as they have their own bodies which are made out of divine magic.

BC pretty much answered everything else I was going to say.