The debate about exactly when the main rotor rpm will fall to a point where it is irrecoverable is impossible to resolve because all stalls, fixed wing or rotary wing are the subject of chaotic behaviour. Put simply, given identical failures on two separate days, one day you will get away with it, another day, you won't. The exact point that a stall occurs is dependant on the angle of attack, air density, humidity, temperature levels, wind gust levels and a dozen other factors that could vary. The manuals and red lines painted on the instruments may give some latitude, but they are the values that have been tested and proven to work reliably for the "average pilot".
I remember years ago expessing surprise at seeing the police helicopter refuelling at a local flying club and doing a rolling takeoff. Apparently with the temperature hovering around 30 degrees centigrade, they couldn't take off from their base, only about 250 feet higher but needing a vertical departure with the tanks more than a quarter full. So clearly the weather can have huge effects on performance and I have to conclude that rotor stall would occur at a higher rotational speed in hot weather than on a cold day as just a single practical example of the variables that have to be taken into account.
Naturally fixed wing aircraft have similar limitations, but they are generally along the lines of make sure you have enough runway to get airborne!
