Lol well I'm still scratching my head on it, so I thought getting rid of all the aerodynamic razzle dazzle and making it a plain buoyancy/regen problem might help. The first thing that jumped out was the pressure change is definitely working against compression/expansion.
For a gross approximation, I would assume 100% regen efficiency in compressor and winch and battery, and ignore energy in reeling up slack/piston friction etc, just a pv=nrt and energy available in ascent and energy lost in compressing at top and expanding at bottom (less force for the same distance), with some arbitrary ascent height, and assume the cylinder/piston is insulated and neutrally buoyant when fully compressed at the bottom.
I don't think there is free energy there, but havent done the math...