No one really knows what Bahamut really is, and the Zeus/Satan sealed him because they weren't strong enough to kill it outright. Fava actually had Bahamut's own barb, so he was the closest to killing. There's no basis to believe whether or not Bahamut would come back based on history. On the other hand, Fava was so damn sure because the tail was still there.

I find more sense in Fava thinking

"My tail's still attached. Amira and Bahamut are still alive somewhere" than

"My tail's gone, but Bahamut must be alive somewhere."

And if Bahamut is still alive and never died, Amira would be there somewhere too.