If the pilot did lower the collective.....why would the crew report there was no collective control response felt or observed.....to the point they were convinced there had been a loss of collective control?
Not challenging your suggestion....just trying to correlate what has been reported to what you suggest may have happened.
Certainly if both engines were at max power.....lowering the collective would not have assisted in controlling the Rotor RPM but you would think they would have known whether the aircraft was responding to a collective movement by observing a pitch change in the blades or felt a reaction.