Polyhedral is fine. I'd want ailerons for sure, even if they were only on the inboard section, which wouldn't give them ideal leverage, but you could make them flaperons and save the weight of the extra servos for separate flaps. I'd stick with the Clark Y idea for simplicity - you can use wing area for more lift if you need it, instead of undercamber. Built-up ribs would be great, but a whole lotta work. Foam is decent at resisting compression, but lousy at buckling and bending, so I'd be careful about foam ribs, but they might be OK if well capped and thick enough.

I don't know how light they would be--you would have to use a lightweight adhesive, but I wonder how well a foam rib/balsa sandwich (and capped on the edges in the open bays) would work as a rib. That would take full advantage of foam's compression resistance, and be simpler to build than built-up ribs. You could still put lightening holes in them. 1/32" balsa on each side should add tremendous strength to the ribs, and they would be very stiff because of the thickness of the foam (1/2"? 3/8"?).