I hate to sound a clever dick, but until the FDR is read and there is evidence of an asymmetric condition, then we are all guessing what would have caused the final dive.
That said, on numerous occasions as an instructor during simulator training I have "arranged" a clutch failure in the 737 Classic simulator at certain phases of flight. Sometimes this would occur as TOGA was selected at the start of the run, and other times while the throttles were idle during a clean descent. Then when power was needed, only one throttle would move. Where the students have been briefed beforehand of the symptoms of clutch failure on one throttle and the recovery actions (simply use the throttle manually) we found some students would quickly take corrective action before the flight path got out of hand.
On the occasions where students had not been alerted to an impending clutch failure during the preflight briefing, there were some who recognised there was a throttle problem and used normal airmanship to rectify any roll or yaw. There were others who seemed to have been so bemused that they failed to apply immediate corrective action until it was too late to recover. Read Post 310 as an example. So much depended on the flying ability of the pilot. It was those pilots who were automation dependant who were the bemused.