Lighting is tough, something I often struggle with and often just 'punt'. I believe there are a couple of tutorials on lighting (or at least some recent posts).

A simple (and hardly the best) way would be to take a large soft, low opacity, low flow airbrush and 'plunk' it down over the magma pool.

As far your sphere not being spherical enough. Do you really need to worry about the ceiling? The room is round enough and a fireball isn't perfectly symmetrical anyway, is it? I think it looks fine.