Hey Graham,
I like your sketches! The vents in the rear fenders and apron is something that Remmele has done to his cars for a long time also and I think the reasoning behind that is strong. It will surely help some, same as it would on the rear of the front fenders.
Not so sure there would be downforce as such on a rear closed bumper though as turbulence behind the car is probably very high. I do agree they act as parachutes otherwise. For that reason I filled mine up with expanding foam (PUR)
Walter