Having thoroughly read through the reports and reviewed the photographs, I find nothing inconsistent with the aircraft being banked at an angle of 90 to 100 degrees at the point of impact with the railway embankment.
The primary purpose of the accident report was to establish the cause and given the significant disruption and lack of flight recorder data, the investigators carried out a thorough investigation given the techniques and resources available at the time.
Given that the aircraft was at low altitude and climbing when the aileron cable detached, the description of the aircraft entering a continuously increasing bank and turn, before stalling into the ground is entirely consistant with the photographs of the scene. There is evidence that at least one control column was commanding a full left turn, presumably a full left rudder input was also commanded and the crew appear to have been attempting to control the turn with engine power, but simply didn't have the altitude to recover the situation.
I don't find anything in the official reports that suggest the crew acted inappropriately at any stage and there are definite indications that they fought to control the aircraft attitude all the way to the ground. The final radio transmission gave some pretty vital clues to the crash investigators and that is unusual in modern accidents where the emphasis is to concentrate on flying the aircraft. Perhaps one advantage of having more than two crew members in the cockpit?
The accident wasn't ever going to be survivable and I can't agree that the aircraft bank angle did anything other than continuously increase once the control cable separated.
Had the control cable separated in a high level cruise, it is possible that some measure of control over the aircraft attitude might have been regained, but I suspect that a spiral dive would probably have been the final outcome.
Given the high speed of the impact but only a modest low nose attitude, the fuselage would be expected to tumble and break up as it slid along the ground striking trees and other low level obstructions.
As far as I can tell, the crew did everything right, they just ran out of time and altitude. The official reports were as thorough as they could have been for the time. There doesn't appear to me to be any attempt to cover things up or hide any failings. It is clear that short cuts were being taken with maintenance operations, but no more than with any other similar organisations at that time. A lot of pilots and engineers had cut their teeth during World War 2 when the emphasis was in getting the job done, not getting all the boxes ticked.
Different times and different attitudes.
Sorry for your loss, but you should be proud that your dad kept trying to fly the aircraft right to the end and he was doing all the right stuff that he should to try and recover given the circumstances.