Not being a suitably qualified pilot, may I ask: are the condition levers moved to full feather as part of the normal post flight procedures? If so I wonder if a partial explanation of the erroneous selection of the condition levers was that the right seat pilot had already done that action twice that day so it would not feel as inappropriate as it otherwise would, muscle memory being another part of the equation.
All this assuming that the final report does indeed conclude that the feathering was manual and not due to some mechanical failure.