So when in the Pegasus Project facility, Aquarian expanded his null field over the entire building. For reference, this is its size:
https://i.postimg.cc/vmCbS2FJ/M2-N1-053-033.jpgNote that it's completely underground, and needs machinery to pump air etc.
As he did so, he was shutting off everything. Even guns stopped working:
https://i.postimg.cc/CK9Fdh7B/M2-N1-V1-058-013.jpg
Including life support:
https://i.postimg.cc/MGL79Fb2/M2-N1-V1-058-016.jpg
Note how people were unable to breathe, choking and collapsing - even metahumans like Ben Grimm were starting to struggle. But Aquarian was fine. So on balance, I'm pretty confident he doesn't need to breathe, or at least doesn't need oxygen. He DOES seem to have a circulatory system, however, as he was strangled before. Comics, eh?
As for your questions, yeah, the solar flare - or at least, the portion that would hit his field - would dissipate. Imagine a force field around him, like in my scan with the TP fog....any energy that is above a certain level, just ceases to exist.
With regards to the propelled air, my reading of it is that if the propelled air is above a certain level in kinetic energy, that energy would be nullified - just like the concrete beam. It just.....floats away.
What else has it failed against? Marvel Zombie Deadpool bit him (and he subsequently healed himself, something that no one else has ever done to my knowledge), he has been strangled, and Cap has aikidoed him before. The strangling and the aikido, was when he was a pacifist hippie, so I'd almost discount them - judo throwing WWH is easy if he's just standing there, not fighting you, after all.
So I.....guess Goku could use his speed, bullrush Aquarian and stop JUST before the null field, and then slow down to bite him at (relatively) human speeds? Bearing in mind he doesn't have the bonus of a virulent zombie plague, nor an amazing HF (Aquarian blasted right through him with his energy blasts after the bite).
Edit: btw, that tactic espoused by cdtm would fail. Why? Because in his immediate vicinity, there would be no vacuum. Air would be normal.
As he expands the field, more and more air would be available. Assuming he needs to breathe, that is.