When the airflow separates, it stagnates, and the local flow velocity drops. When it drops, pressure rises, and what you have is an increase in pressure, as well as burbling air, aft of the flow separation. It's not a vacum back there, it's just not benifiting you.
So…how can an aileron do it’s job in just such a situation you describe? With flow over the aileron separated long before CL max, as you say, and then if the pilot provides an input to deflect the aileron to the up position, how does this separated airflow provide a downforce on the wing?
Again, not talking of any specific wing, just an airfoil with these characteristics. And I appreciate your replies, I’m just trying to better understand the forces that could be present on flight controls at, or just past the stall. Maybe I should have just asked that instead.