D3, that simulation is truly excellent! I hope it will be wielded on a regular basis to provide that extra level of technical expertise.
(I can't remember if you posted a link)
If your Prandtl-Glauert adjusted AOA max is based on static Cl plots, then retreating stall will in practice be much higher. This is because when the blade/wing stalls there will be a seperation vortex as the rotation is lost (opposite of the starting vortex on a fixed wing takeoff). This seperation vortex produces high speeds over the blade while being shed, so that transient lift is produced for a while. I can't describe this in more detail, but i gather that the vortex shedding can provoke flutter if the blade natural frequencies aren't tuned. As you say, vibration is bad.
But either way torque goes up, so power required goes up. The machine cannot achieve level flight. I can imagine Nr droop as you comment.
As FayeDeck indirectly comments, in a teetering rotor retreating stall would cause massive flapback to limit speed. I imagine this happens beyond Vne, so should remain a theoretical discussion. If Nr droop caused the retreating stall you suggest, wouldn't that just limit speed?