Rotor stall
By looking at data and running more simulations I got convinced that Shawns explanation is right on (that in it self could have been stated without doing the calculation and just trusting Shawns experience..)
What I see is that depending on load say between 75% and 50% usefull load the retreating blade stall will set in rapidily at any significant forward speed at rpm's below 80%. As Shawn explained this will create a blow back, and even moving cyclic fully forward the heli will slow down getting into a vertical autorotation.
I see that at low speeds (say 10 knots) RPM of 70% down to 65% are reachable before main rotor stall sets in. Up to that point coning remains reasonable (below 10°) but of course we run out of steam very rapidly beyond that point and even before the stall will hit the ground with significant vertical speed.
m2c