And even at a Np of 1586 the engine would be driving the prop
Maybe, maybe not. All you can tell was the prop was rotating. For the zero thrust setting of 100/1,600 and using recommended One Engine Inoperative Climb speeds the engine is producing no prop thrust, but consuming 30.46 shaft horse power just to rotate the prop. If you were to reduce TQ to 10 ft-lb and the prop maintained 1,600 RPM, that means the slipstream is contributing 27.4 SHP. What the prop RPM at zero TQ and recommended One-Engine-Inoperative Climb speeds I have no idea.