I notice that you're using a molded eliptical wingtip style. From the flight videos I saw I suspect this is part of your trouble. Much of the unpleasant flight characteristics appeared to be in a high angle of attack, and I believe that your wingtips are stalling. Extending your ailerons further toward the fuselage put more area in an unstalled portion of the wing further increasing their effectiveness.
I think that either building some wash-in to the wingtips or making them under-cambered like the FT Spitfire will help a great deal with this aircraft's flight characteristics.
That makes sense, thank you for your insight!