The videos certainly demonstrate that flight is considerably prolonged with the application of left rudder to counteract aileron deflection.
The question of the aileron positions during flight can perhaps be simplified somewhat by considering that the aircraft maintained a constant turn to the right. This suggests that the aileron position at the point the cable broke was at the very least maintained.
Application of left rudder failed to rectify the situation, so on the balance of probability, the ailerons appear to have moved somehow to a position where they became even more effective at rolling the aircraft despite the application of full left rudder. The aircraft would have been flying in an unbalanced sideslip. Would the disrupted airflow over the wings have been enough to move the ailerons, sucking them further from a neutral position?
It is probable that the engines on the right wing would have had the power pulled back at some point in order to try and get the wing to drop. The left engines would be left at full power to try and prevent an incipient spin developing which would have led to an immediate crash.
As you say, the closer you look at all the variables and interactions that could have been involved, the more difficult the problem becomes in arriving at a credible solution, rather than one that just happens to fit.
This is why I suggest that the flight needs to be considered as a series of events with the aircraft reacting to the failure and the crew's attempts to corect the problem.
Aircraft takes off a bit long and low on the runway, not necessarily a problem.
Aircraft begins a standard banked departure turn.
Aircraft climbs in a banked turn.
Aileron cable breaks.
Crew are unable to adjust the angle of bank using ailerons.
Bank increases.
Up elevator commanded to maintain height?
Progressive application of left rudder to the limits.
Further movement of aileron due to disrupted airflow over the wings in sideslip?
Aircraft starts to descend.
Crew reduce engine power to right wing engines, maintain full power on the left.
Does the application of differential engine power move the ailerons still further out of the neutral position and increase the bank angle?
Aircraft continues to roll and descend.
I have a hunch that doing the right thing in reducing engine power on the starboard engines while leaving the port engines in full power may have resulted in the ailerons being moved out into a more effective, rather than reducing to a neutral, position in the slipstream. Flight training would have taught that applying more power to the port (left) wing should raise the wing and turn the aircraft to the right. So while the flight crew reacted correctly and in accordance with their training and experience, it is possible that the "correct" course of action would have been to completely and counter intuitively reduce engine power, particularly on the port wing first. This would inevitably have resulted in a crash landing, but just possibly in a more wings level attitude.
As I say, this is just purely supposition and it is possible that the crew did finally take a counter intuitive approach to control of rhe aircraft when it became obvious that a crash was inevitable. That could certainly account for the apparent reduction in bank angle before the initial impact, which is what you have been trying to explain all along if I have my facts straight.
I doubt that these possible elevator control inputs, aileron positions, engine power settings and rudder inputs can be duplicated using personal computer flight simulation software, but approximations can be made to test the ideas. As you have already demonstrated, just applying a hard rudder against increasing bank angle dramatically prolongs the flight.
I think you are making genuine progress in explaining what actually may have happened. You are certainly building a fairly strong case that tends to support and explain your version of events.
I don't think the accident investigators at the time were at fault in any way. They had to work with the resouces available to them at the time and no doubt with considerable commercial and government pressure to produce a result quickly.
You have the benefit of being able to review and model potential scenarios over and over without any constraints. I would be surprised if you don't eventually arrive at a better solution to this tragedy, one that demonstrates that your theory is correct and also agrees with the results recorded at the time, if not the conclusions.