The gear box is having no more than the maximum torque allowable go through it.
From my understanding of it, SHP is effectively the torsional energy being applied to the drive shaft via the power turbine and reduction gear. As you said, setting max Trq limit of 1970 with 1900rpm may not overspeed any component, but based on SHP definition, you have increased the stress imparted on the prop drive shaft. I would assume you will find yourself in all sorts of problems if it where to be overstressed and fail.
NG and ITT are indicators of the gas generator. That half of the engine could very well be within limits while your trying to make your prop depart the airframe.