The more I think about this there are two things that concern me...
one, 15 foot of water is not much to work with. I have found the thermocline to be very fragile and can be disturbed very easily. I have my diffusers not deeper than 6 foot deep in my 10 foot deep pond and have never witnessed a thermocline while the aeration system was in operation. So, creating some extra lift above your system may pull deeper water up with it. If you have to choke it back to avoid this, it will throw the lower works out of balance requiring the air input to be reduced as well. (reducing the affectiveness all around)
two, the balance of the system will be critical to maximize the effects and utilize all the air you can pump to it. The size of the bubble vent tube is very important, too big and it will draw water up with it and too small and it will cause the air to burp out of the water outlets at the bottom. Now, throw an air control valve in there and you have to adjust the air input to the diffusers to balance the output.
I don't think these concerns are killers, but be prepared to do plenty of testing and retrofitting (and maybe abandoning the bubbling of the upper waters all together). Option "B" is much more forgiving in that you could just balance the size of the inner tube with your air input to move water and then start drilling small holes in the outer tube at the top to allow some of that water to escape into the upper column. I think the advantages of this are minimal and it reduces the affects below the thermocline.
I wonder how much water I would end up releasing above the thermocline as the bubble tube would effectively become an airlift. Would a short tube be better than a longer one? That's where trapping and separating the air would be key. How would I strip bubbles out of the water?water.
Once again...a balancing act. The closer to the surface that the bubble vent tube terminates, the less movement of water is realized hence the less of an effect on DO. The lower in the water column that it terminates, the more water movement AND the higher the possibility of disturbing the thermocline.
The stripping of bubbles happens in the top of the barrel not in the bubble vent tube.
I am also planning on putting in some static mixers in the vertical tube to increase contact time with the air bubbles and mix the air and water.
This is a very good idea and, again, a lesson in air and water flow balance. The more static mixers that are installed the slower the water and air moves and the larger that part of the system needs to be to allow it all to move through without the air backing up and burping out of the bottom of the diffuser intake funnel.
I have a general idea of fluid dynamics theory, but there is no way that I could engineer/design this system on paper with the hopes of it working out of the gate without spending many, many hours of calculations (even then, it would likely need modifications once installed). With that said, I would feel much more comfortable building option "B" because of it's minimal number of components and restrictions. If you have actual drawings with dimensions, that's a different story, but building it from the simple diagram leaves alot to figure out in the field. Be prepared to install, test, remove, modify, re-install, test, and so on. Don't let me sound like a "nay sayer"...it would be a very cool project, but a complicated one.