I would rebuild the aileron so it is stiffer and would definitely sheet it, maybe even with 1/64" ply on 1 side. Make sure the torque rod is well inbedded in hardened wood. I like to use epoxy "bearings" for torque rods instead of using tubing now for the least slop possible.

I think having control surfaces that are thicker than the wing would increase buffeting? Seems counter-intuitive.

Decreasing surface area of the flap is obviously going to help, I would look at other planes in your class and see what they get away with for roll control.

Funny how you helped me out of a flutter problem 2 years ago with my Nelson powered delta, that model is still with "the living", needs to be exercized soon.