Since it runs flat until you get some speed I now suspect one wing panel isn't set the same as the other probably the right one has gotten a warp, leading edge high at the tip or the left one has leading edge low. It's definately rolling left at speed as opposed to turning left. It's accelerating straight pretty well so backwards prop or not, thrust isn't the issue. Crooked wing to fuselage alignment just causes odd loops and slow turns towards the retreating side. Doesn't look to be tip stalling, it just a nice fast roll to the left at flying speed. You could attempt a flight at full right aileron trim to see if that changes anything but check alignment first.