Spuis
There doesn’t seem to be any data for power lever angle on the FDR. However the propeller low pitch parameter indicates that the beta mode was engaged (or available?) for approximately 16 secs; this appears to be coincident with gear lowering, the reason given in the report.
Thus as your post suggests the mechanical lock was lifted and the secondary device was not available due to some other cause yet to be determined. I assume it to be normal practice for the crew to select flight idle, relying on the mechanical lock to prevent beta range in the air. The crew may or may not have been aware of the design weakness that lifts the lock for 16 secs with gear lowering. If the crew believed that the aircraft was modified then the need for awareness during gear lowering was not required. There does not appear to be any evidence that the crew lifted the lock; I could accept that the timing of the low pitch indication with the gear down is circumstantial, but the engine parameters indicated idle, descent was commenced, and flap selected much earlier, thus I hope that the logical conclusion ties the lock to the gear.
Some crew may have detected that the power levers were displaced further rearward than usual, I do not know what the physical dimension is, but many crews in high workload situations have missed similar cues and also ‘forget’ any awareness of design weakness if applicable. Thus at best it may be concluded that the crew were unwise to continue the approach with changing circumstances, but I suspect that there are some of us who have done something similar, and probably all of us wish that we had the wisdom of hindsight after poor judgment.