Ok, I agree that auto RPM can be 'set' - however it is still controlled by the pilot. one of the things you are taught is rpm control with the collective -especially during turns, flares etc. I don't think "the RPM was set wrong!" is a valid excuse.
Anyway, from 50' unless you are some kind of Jedi you are going to lose a lot of rpm if you wait 1 second, and in the 5 seconds or so it takes to reach the ground you'd be lucky to get it back to 100% let alone overspeed it.
Excess rpm can always be reduced to decrease ROD - of course you have to pull collective before you hit the ground!
Almost forgot - R22 doesn't have negative pitch. There is flat pitch, but in autorotation airflow is from below so AOA is positive.