To my beliefs the blade angle of Atr42-500 is also linked to the PL, so if in case u move the PLs , it also changes the Blade angle even if the engine is failed (FCOM1 CH16) which would increase the drag to a significant value when PL is moved backwards towards the FI position - making the blades FINE and in such a critical phase as takeoff or a go-around an increase of drag may prove to be lethal. so 45 degrees PLA is the min safe angle below which significant drag would be induced by the failed engines prop hence reducing climb performance n increasing the VMCA above the ones calculated for a feathered prop.
There are blade angle and speed relation tables given for simulated failures in FCOM2 procedures and techniques that can be referred too for info purposes.