My nose comment was about the square nose causing so much drag. I dont think it is that fatal to have a square nose 'cowled' only by the motor.

There was no mention of elevator deflection in his post, just ineffective yaw. If the elevator is suddenly stalling when rudder is used, it must be losing airspeed, or the elevator would have been stalling before rudder was applied. Doesnt sound like it was already nosing down before the rudder was applied, so it sounds like the rudder is bleeding airspeed much the way a good air brake would do.

He can try less deflection to check, very easy diagnostic. If it turns out that deflection is stalling the plane, then he can remedy that with a hog of a motor, al la 3D, or reduce the throw and let the wings do some of the flying!

He should also remove the turbulance caused by the lack of a smooth canopy, that will indeed make the rudder less effective. A simple wedge in front of the step-up might suffice.