Hey orby.. great job so far! Joined just to follow your work
Anyways, as I was reading about your tail, the first thing that came to mind for your bottom side detached flow problem was strakes or fences. These help keep the air from 'circling' and force it more into a straight line. If it does spill over, it'll 'trip' over into a vortex. This idea is already in use on autos to help remove the air from under the car (rear diffuser). Cheaper and easier to try than redesigning the tail.
grrr - cannot post links or pics... Just google 'rear diffuser'
You are correct on needing to have more outlet for air in the belly pan. I believe what you did was trade under carriage drag for increased frontal and cooling drag. Try to think of how to get any air
smoothly out..
Good Luck!