It shouldn't be too difficult to eliminate the wing warp. You basically have a "D" tube wing design, except I don't see any shear webs between the spars. Shim the wing flat or with what ever amount of wash-out you want, then glue a vertical web to the back side of the spars between each rib, using 1/16" balsa. That will make the wing structure a D tube which is very resistant to further changes.