I think it’s possible, but it would have to be a very advanced airbender and I don’t think that airbender would get a better control of it than a waterbender would. I get what you mean on the “bending the air in the water” but from what we can see in the Avatarverse, it’s “bending the element in a body” like how bendling the blood in one’s body, bending the water in the vines, and bending the earth in the metal.
That’s a good theory, I would really love to see more of the air nomads’ past and their culture. As I said before, there’s darkness inside ourselves and its in our choice to let it out. Similar to how Katara has a dark side (The Southern Raiders) but she was able to control herself in the end. If Air Nomad’s have this power, this must be only used for extremely necessary purposes. (Like how Gyatso was able to take on many firebenders by himself)
idk maybe in the end the high level technique of airbending is actually the marble trick.