Pretty much everything you stated is often why people mistake them for having draconic ancestry in the first place. Still doesn't mean Au Ra (or anything with similar traits) = dragons. Au Ra existed on the Source before Middy ever showed up and official sources (doesn't get more official than the game itself) are pretty clear that they have nothing to do with each other. The game also makes it quite clear that dragons are from another planet; hell, we visit a partial memory of that planet in Endwalker. For Au Ra to be descended from dragons the devs would need to throw lore out the window.
EDIT: I don't recall that group of Au Ra in the Ruby Sea being stated anywhere to have some super natural ability to breathe under water...? What does that even have to do with dragons, given that's not something associated with them, really? I could be forgetting something (insert insomnia complaint here), but even discounting that, we, the player, gain the ability to breath under water via a blessing from the Kojin, so it's not unheard of. Also, the "Drahn" thing is very likely meant as a joke, not some definitive confirmation.
EDIT again, don't post when low on sleep, you fool!: I doubt any of Middys' kids can traverse the Rift (space) as he can, probably relating to his extreme age and how the older the dragon= the stronger they get. The game goes out of its way to remind you how powerful he is (was) yet tells you traveling through space taxed him severely, and he's never quite been the same. Not enough time to recover, I suppose. Each time we've seen/heard of him, it's about him waking after a long sleep to do some stuff (fight off the Garleans at Silvertear, for example), then immediately return to sleep to restore himself... none of his children seem capable of the same feat (Middy has "died" how many times, now?), so again, I doubt they can travel through space.