There didn't seem to be any stress on either pilot when the error was made - the PM was seemingly distracted, not stressed; the PM was simply not paying attention**. The flight was smooth enough, the day was nice, the weather was clear, the runway was in sight. The confusion and stress came later when they noticed the airspeed had dropped.
We don't have enough information for that call just yet, that is to determine what the workload was like. Circuits, especially the latter part of downwind, base and final is very high workload in a large aircraft, the PF is very engaged with positioning, vertical profile, speed and configuration. In this case the aircraft was at very high weight, we known that as it was full, so the speeds would all be increased and the PM in a training environment would be keenly watching everything the PF does. I would say the PM was at high workload to give them the benefit of the doubt, add in the PF being off profile, wide or close, a bit fast or slow, vertical speed not to the PMs liking but within the acceptable range and so on... then add the PMs poor cross cockpit visibility of the turn to final so straining to see alignment you might see an urge then to keep the eyes on other things than something hes done a thousand times before like extend the flap.
We all make mistakes, I just hope mine are limited to simple easy to fix things, in understanding that we all can err, we can then move forward and build up our own defenses against them.
As for more mechanical locks, as said above there are then brings in issues when you really want to feather the props expeditiously. You don't want the asinine computer frustration of everything confirmed with "are you sure y/n" "are you really sure y/n" "and one last time are you really, really absolutely sure you don't want toast? y/n".
I think more bells and whistles would have helped here, the idea of aural feather alerts is a good one IMO. Is it possible in this case as well, if it was some weirdly ridiculous mechanical problem, that if they were alerted earlier to the feathered props that they could have altered flightpath to land on one of the runways nearby? We still don't know why the props feathered exactly, it does look most likely the PM did it, but there is a lot more data to be released before its certain. Then there is also the remote possibility the PM did it intentionally for whatever reason.... It may be we never really know why.