Although there was no aileron control, the decreasing bank angle, the radius of the turn and the relatively shallow descent could be explained by application of left rudder, probably to the stops. Application of full left rudder would have shallowed the bank angle and decreased the turning radius thus fitting your hypothesis. Still having elevator control and throttle control the crew just might have been able to and quite possibly did set the aircraft up for a semi controlled belly landing. Had they had another hundred more feet of altitude or if not for the railway embankment they might have made it. It would still have been an ugly impact but possibly a semi successful one with some survivors.
when the depth of the failings of the NW maintenance staff during the replacement of the aileron boost unit on N137US became apparent all eyes and all hands rushed in that direction and the impact sequence part of the investigation was left hanging. At that time everything about the Electra was under major scrutiny and a severe cloud of suspicion hung over the aircraft. Coupled with the serious failures of the NW MSP maintenance and inspection staff uncovered during the investigation I can easily envision a scenario where all eyes and attention were diverted in that direction due to the potential for further safety concerns. The downside to that shift being an overlooking of a possibly brilliant and heroic bit of flying on the part of your father and his crew.