There are no separate "tiles" for interiors right now. Maybe some modders will create them. Right now you will have to create dungeons, etc. out of meshes like rocks. It looks very natural, but it is of course some more work. It is the same for interiors of houses. There are only "complete" interiors right now you can use. Crowded city streets are no problem.
Sure, you can. There are no "doors" in interiors, there is just a free place for the modder to add doors. So you could of course add several houses together.
Existing building parts aren't suited for modular level design. But if you're capable of creating your own, then there is no problem.
Well if you are experienced enough in any 3d modeling program you could always create meshes there and put them in the red kit, you may even port some meshes from skyrim(with permision or credits of course, but I dont know if this would be possible, just talking with a bit of intuition.