Which of the versions will yours be most like, "A" or "B"?...
OK, I went back and see that you are looking at "A" to mimic....
What would happen if you terminated the bubble vent tube below the water surface, maybe with a big flow control valve? The air that the diffuser releases would travel up the bubble vent tube mostly free of water and be released such that you get some water movement near the surface.
The control valve would allow you to choke off some air flow in case the lower section did not create the air pocket at the top of the barrel like you want for water separation. The side effect of this migh be that you get water sputtering out of the bubble vent tube at the top. Choke the valve down a bit and the pocket is created which helps separate the water from the diffused air.
