Crab
Unlike a 341 the NR in a 500 is very very lively. You can use the technique I describe even counting to 2 before rear cyclic !
The 500 when you drop the lever seems to drop its nose way way more than any of the other types I fly. So you are able to use the lever as speed decays i.e. slowly putting it down drops the nose alarmingly which means you flare back to maintain attitude and speed which also obviously helps the rrpm stay in the green. I hear what you are saying but the 500 seems to autorotate like nothing else ( well having done an EOL in a Scout with 3200 fpm ROD Ii might take that back )
Not sure on the 600 but auto Vne in a 500 is 130 kts, a 600 will easily get to 135 / 140 kts. So if they were doing an auto from this speed I can see why they hit the tailboom especially with an inexperienced instructor !