I did see a translation of an interview that listed Bahamut as one of the seven wyrmlords in Heavensward along with Tiamat and Nidhogg. Judging by that Id say that the original Bahamut was indeed one of Midgardsormr's children. My guess is that he either died and a primal form of him was summoned or he was turned into a primal.

There is nothing saying all 7 celestial dragons are still alive.