Post 19 suggests the coffin may have moved forward and pushed the control column forward.
Highly unlikely I think.
People have been hauling coffins around the Territory in the back of C210s for 30 years that I know of and probably much longer. I haven't heard of problems.
Coffins empty or full are relatively light ( not like a loose,full 200 litre drum of fuel that once caused a fatal) There were 2 pilots on board so between them the coffin could be quickly pushed back.
The main evidence that the coffin wasn't involved is that the aircraft impacted flat ie parallel with the ground. If the controls were pushed forward as suggested then the impact would be more vertical.