I was an instructor in the 76, and have quite a few hours in it. It is difficult to diagnose on the basis of so little info, but I think you guys are on the right track. It looks like a jammed or broken tail rotor cable could have been the culpret.
With a powerful tail rotor intact, but driven to full angle, a slowdown from cruise will make the aircraft spin, and the collective driving is a symptom of the pilot's foot trying to push pedal against the jam, and instead back-driving the collective. The nose pitch up and such are artifacts of the strong yaw to pitch coupling of the airframe, so that a left yaw at high speed looks like a strong pitch up.
I wonder what maintenance was performed in the several hours before the accident, if the tail cable was touched, it could be a sign.