Relationship between Bahamut and Midgardsomr?

[treuhavik]

Well, one thing to bear in mind is that a Primal doesn't necessarily need to look exactly like the historical figure they were based on. The original Shiva, for instance, was most likely not a giant, blue, floating woman. Bahamut the primal might only superficially resemble Bahamut the original.

This.

I'm not sure it was ever stated that Bahamut was one of Midgarsormr's seven sons?

It was confirmed during the last interview that Bahamut was among the Seven Celestial Dragons, making him Midgardsormr's offspring.

One thing to be careful about when speaking of Eorzean geography is that the Calamity rearranged the land somewhat. The Crystal Tower could have been built relatively far from Silvertear Falls, but after the land shifted due to an artificial moon crashing into it and then a giant dragon exploding like a nuke, the two locations could have been pushed together.

The Crystal Tower did move, actually, but not far enough, I think, to avoid it's detection by Midgardsormr.