Like I said, I'm not an expert...perhaps someone else here will chime in, who knows better.
A useful analogy would be a child's swing vs. a see-saw. The swing is like the conventional hang-below weight-shift design. You can sit in the swing, and via weight shift only, get the swing to oscillate. But if you stop pumping, the swing will return to the stable position, hanging directly below its attach point. If Peter Perturbation comes along and shoves you (the cad!), it'll displace you from the stable position but you'll quickly return to it without further action.
The pilot-on-top weight shift is like the see-saw. Imagine standing on it and straddling the pivot point. It doesn't take much to hold position. You can even lean over a bit (a bit!) and hold the plank in a slightly tilted position. But too far, and that end will come crashing to the ground without you being able to stop it. The same thing will happen if Gary Gustfront sneaks up and pushes one side up. If your reflexes are good, and you see him coming, and he doesn't push TOO hard, you might be able to recover. Odds are, no.
And recall, your design uses weight shift for both roll and pitch. The see-saw in this case doesn't have just a hinge across the middle, the whole plank is resting atop a cone. Any motion...fore and aft, left or right...is going to displace your position and will require a countering action to keep things from going goofy.
Yes, increased dihedral will help. If you run a computer program to determine how MUCH dihedral would be necessary for stability, it'll probably give you a value that puts the center of lift ABOVE the pilot. In other words, it becomes a classic weight-shift design.
There's a lot of argument about the need/desirability for stability. Some pilots believe that neutral stability produces more responsive aircraft. However, one must not forget that the Pitts Special is a fully Part 23-certified airplane...no one complains about ITS stability.
The problem is, it's my believe that this "inverted" weight shift arrangement produces negative stability. It'd require a very good pilot to keep it under control, and it'll probably require their full-time focused attention. Not fun.
I do like your design, though, Arthur, and look forward to hearing how your RC tests come out...with whatever configuration you end up with.
Ron Wanttaja