You have investigated everything except stiffer control horns. The molded plastic ones that come with the kit are kinda limp and if they are not installed over a hard-point, they can dent the balsa that they are sitting on and work in some slop. Trim out some control horns from3/32 ply or carbon fiber sheet. Slice full depth through the aileron, and epoxy the new stiffer horns in place. Make sure all the geometry matches wing-to-wing, that the linkage hole is aligned with the hinge line, and the hinge holes have the same height in both wings. Heavy-duty servo arms, and beefy pushrods complete the triad.