[Amenadiel]

1.- there is a mod that forces a style upon a specific domain (different style different domain it is).
2.- you should use CES cookie cutter to do a single login/logout
3.- you can use template conditionals on each style (hence in each domain) on the template forumbit_level1_nopost (not sure) to display only certain categories.
4.- You still have to work out a way to redirect or rewrite wrong combinations (i.e. linking to threads of forums that "belong" to another domain) you will need to do this with a plugin.
5.- Above all things, you should buy as many licences as virtual sites you'll have.