If several million tonnes of rock can be suspended indefinitely in midair from an overabundance of air aether why is it difficult to believe that would also supply an abundance of air to breathe?

Breathing underwater can be explained in much the same way. The aether from the ritual is converting the water to air or stripping the hydrogen from it leaving it safe to breathe. Our natural carbon dioxide exhalations wouldn't dissipate immediately in the bubble so we don't need to worry about oxygen poisoning from breathing too pure of an oxygen. If the above is possible then mitigating the pressure exerted by expanding bubbles of gases trapped in our body would be a relatively minor thing to include as part of the ritual. We're repeatedly shown that aetheryte crystals have the ability to create/warp/manipulate energy and elements throughout the game so this is by no means a stretch of the imagination.

TLDR, magic.