Modern free turbine turboprops don't have NTS protection as such. The ATR has overspeed protection through the PEC which will eventually flame out the engine eventually if a certain RPM is reached. That is it just moves the blade angle coarser until it reaches the high pitch stops (78ish degrees) and then prop RPM will increase until the overspeed protection kicks in. More likely the near feathered prop feathered when it hit the water and failed, or the impact at high rpm forced it there, the other prop would not feather as part of the autofeather logic as that engine impacted and failed hence it was half travel or so.
At the speed this ATR was travelling as well as being configured the prop would be well within the normal operating range.
Because of this during fault finding you should be setting a low torque above flight idle position to simulate zero thrust if its not an actual failure, otherwise the prop will generate a lot of drag from 'windmilling'.