At the time of the accident I believe only a single cable loop was used in the aileron circuit. The design was subsequently modified to a dual cable or redundant system in all commercial aircraft thereafter. It was one of those defining moments in aviation history where lessons were learned and fixes put in place immediately afterwards. any engineering drawings or references post the crash will show a dual system that was not fitted to the crash aircraft.
In this particular accident the cable broke and all aileron control was lost to both yokes. The autopilot system was flagged inoperative and the switches that would have enabled it to be activated were not accessible to either pilot. It is not known whether the autopilot system was actually functional or solely deactivated pending maintenance and an update.
The report clearly states that a turnbuckle was observed to unthread and separate during vidration testing on a rig without a safety wire inserted. I agree that a twisted cable in a sheath behaves in a similar manner to a solid rod but you are making the incorrect assumption that both ends of the cable were secured to prevent any axial movement. This is clearly not the case here. The turnbuckle was free to rotate and unthread, effectively severing the cable loop.
Additionally it is mentioned in the report that the loose cable snagged on other parts of the aircfame during testing preventing a return of the ailerons to a neutral position. It cannot be definitively stated whether this happened in the crash, but fact that it happened during testing indicates that this is very likely.
So the ailerons were deflected to initiate a turn, the cable seperated and jammed, leaving the ailerons with a permanent deflection that could not be overcome by rudder inputs. Although the crash investigation team could only prove a deflection of between 3° and 6° from witness marks, I feel it may have been rather more than that prior to impact. The tail and rudder are pretty substantial structures and should have had sufficient authority to level the wings. However the low airspeed during the initial climbing turn must have prevented full rudder authority being available to level the aircraft. Takeoff flaps were still selected when the aircraft crashed and this may have reduced tail and rudder authority slightly by deflecting airflow downward beyond the wing surfaces.
Had the incident happened at 10,000 feet, might the accident have been survivable? Probably not. It seems likely that the aircraft would have entered a spiral dive and impacted the ground before control could be reestablished.
An incident of this type requires immediate intervention and rapid full control inputs, something that airline pilots are trained not to do for the safety and comfort of their passengers. Survivable in a test pilot flight test scenario but not in day to day commecial operations.