Part of the problem of trying to figure out the realistic physics of the world is that we have no real in-game approximation of it. The physics engine for FFXIV is relatively basic, if it's even there at all. That combined with the fact that the size and nature of the game world is warped for better gameplay makes it rather difficult to understand the 'real world' physics. That being said, it can be generally assumed that most physical laws of our reality exist to some extent, even if they can be hilariously stretched. To go back to the initial post, it can be safely assumed that the blessing of the Kojin provides some way to survive the pressure in addition to the issues with speaking and breathing, although given the relative scientific illiteracy of the Kojin (and Eorzea as a whole) we would have no idea how.

On the subject of Aether though, I'm not sure that we can totally limit it to the conventions of normal science. It seems to be capable of acts far beyond the capabilities of the real world, from instantaneous creation or teleportation of mass to the production of ethereal and unreal creatures. Trying to figure out how some of that might work on a physical level is both fascinating and headache inducing. Even Garlemald's magitek seems capable of breaking physical laws quite easily, so who really knows?