Army AV8TR,
I agree with your point about Nr; seems like there were two problems, one the loss of power (for whatever reason), then the inability of the aircraft to maintain at least min autorotation rpm (see my post about unseating or damage to swashplate/pitch control mechanism).
Just looking at the apparent failure to autorotate all the way to the ground, apart from mechanical damage to the pitch mechanism etc, there have been aircraft lost due to mis-rigging where they can't get enough pitch off to auto. Would take two pieces of the cheese to line up, but then that's happened before too.