So you are saying that downwash alone causes the flow separation. I doubt it especially in an airplane that has long coupled tail location and a horizontal tail that is only a few inches below the mounting height of the main wing. Two categories of causes of flow separation are common: surface contamination is one, and exceeding critical angle of attack is the other. Downwash is not one of them. And please do not go doing spins in an airplane that is not approved for spins, without a parachute, without a drag chute and loaded to the aft CG limit!