We know that Mannimarco originally worshiped Molag Bal, but now that he's ascended to God status as The God of Worms, does he still worship Molag Bal as his lord?

Each Daedric Prince has their own realm, so it's not clear to me if one Realm holds sway over the other. Scroll down to Daedric Princes

http://www.imperial-library.info/content/guide-daedra