As a physic student I cannot say I did not notice all these details. While I try my best to think of a scenario where all these things could make some sense, there are obvious loophole. Considering the size of some structure and the ability to fly of most things which don't seem the most adequate physiology to do so (flying swiiminglike snake from Vanuvanu), what could make some sense is that hydealyn first is a less massive planet resulting in a weaker g force. My guess would be that 80% wouldn't make much of a difference wheen it comes to running or ground battle, but it would allow much taller structure to exist like ishgard. A much thicker atmosphere could also help explain how some animals manage to fly. Lower gravity + denser makes flying akin swimming in a liquid with extremely low viscosity.
A much Thicker atmospher could also help resolve your question regarding pressure underwater.