Sid I understand your attraction to a TR problem theory followed by a deliberate in flight shotdown. However, as Henra points out, there is really only one way to have zero NR at impact. I cannot see how the pilot would choose to chop throttles without already having lowered the lever and at leat prepared for the autorotation.
Seized or jammed engines/gearboxes I believe would lead to destruction of the drivetrain in flight. It would appear that this did not happen.
DB