Mad Dog, what's your information source for these statements? The Curtiss Electric prop we had at mech school was not at all like this. If the governor or the slip rings failed (rare), yes, it would go to full increase, but since it came off a single engine plane (Curtiss SC21? I think), it wasn't featherable. Full increase would be roughly equivalent to a "TO & climb optimized" fixed pitch prop. A modest throttle reduction would get RPM back in the normal range and cruise would be slower than normal, but stalling out of the sky sounds pretty extreme to me.
A much more common failure would be for the electric pitch motor to fail, freezing the pitch at whatever value it's at. Unlike a hydraulic prop, which allows aerodynamic loads on the blades to drive it to the stops when hydraulic pressure is lost to the dome, blade loading on an electric can't drive the motor due to the mechanical advantage of the reduction gear. About like jacking one wheel on a car and trying to turn the engine over by rotating the raised wheel by hand. This changes your constant speed prop into a fixed pitch one, and returns RPM control to the throttle.
If you could feather the prop, the P39 looks like it would be a good glider, with its sleek profile, EXCEPT for that dang symmetrical airfoil. Now with a nice NACA high lift airfoil, I bet it would sport a pretty decent L/D, albeit at an alarmingly high speed and sink rate.