Here is what I understood from that system. Negative torque occurs when the prop drives the engine (windmilling prop). When this happens the NTS valve closes, causing oil pressure to build up and move the feathering valve, dumping oil pressure from the prop piston to the gear case, allowing the counterweights and aerodynamic twisting force to increase blade angle. This increasing of the blade angle then decreases the force of negative torque on the engine (less drag from the high blade angle) and opens the NTS valve, allowing the feather valve to fall back to its normal position from its spring. This process occurs repeatedly (called NTSing) until the pilot pulls the engine stop and feather control to feather the engine.
They also designed it with a NTS lockout system to prevent a windmilling prop from NTSing when on landing rollout. This NTS lockout valve only works when the pilot moves the power lever aft of FLT IDLE, which drains oil from the prop governor piston and NTS valve so that oil pressure cannot build up to operate the feather valve. They want you to be able to feel that you have an engine failure when you are rolling down the runway. This ensures that your power levers (which control blade angle on the ground) will create the maximum drag to get the aircraft stopped if you have an engine failure during landing rollout.
As to your questions.. I am not exactly sure why they didn't design it as a full feathering system. I believe it has something to do with the engine restart in the air. The NTS system pretty much holds the prop near a high blade angle (close to feathered) so the drag is negligible when you have an engine failure and the system is operating.
As for your pre-flight check of the NTS system you are checking the oil supply to the NTS system. On the ground the power levers control blade angle (in the air they control fuel flow), when you increase the power levers you are checking to make sure the RPM stays around 94.5%. It stays in this range because of the oil pressure going to the prop piston is increasing with the increase in power to drive the prop blades flatter. I believe the NTS valve opens slightly during this test to allow oil to drain out. If oil did not drain out the RPM would increase to higher than 94.5%. So if this test fails to give 94.5% there is oil supply problem in the NTS system or the prop governor low setting is misrigged.
