Servo stalling or blowback are not flutter and I've never heard anyone suggest that they are.
I have heard several people over the years attribute what I assumed was a servo stalling as flutter as a matter of fact I think they even called it "High speed" flutter which I believed was incorrect but everybody is intitled to the opinion and since it wasn't my plane that crashed I wasn't the one that needed to justify the cause.
The biggest flaw in that argument is that flutter is not uncommanded movement of the whole model but, as you initially wrote but then ignored, just in a control surface.
I'm not sure I understand what you mean if the control surface isn't moving to cause the whole model to move then what is causing it?
A gap in the hinge line reduces the efficiency of the control surface. How on a mechanically sound control surface can flutter be induced? If the gap allows air to pass thru the control surface where is the uncommanded movement or "flutter"?
Just curious?
Wait so you are saying that the air passing thru the gap in the hinge line creates turbulance with causes the surface to move or flutter...