1. What is the airspeed/altitude/pitch combination that you should not cross- after which you will not have control authority over the horizontal stabilizer??
To get that ball rolling, the surface area of the 731/2 THS is as near as nothing 30 mˆ2, or for those who still like to think in "psi," 46500 inˆ2. In the "Classics" that area increased to 31.4mˆ2, and in the NG/MAX to 32.8mˆ2. (source, b737.org/techspecsdetailed)
From that, someone (not me) ought to be able to calculate the net pounds or kilos of force needed to move the stabilizer against the force of a given ram-airspeed. As modified by the pitch angle of the stab (trigonometry: sines, cosines, tangents etc) to the relative wind.
IF that value can reach as high as 1 lb/inˆ2 or 704.5 kg/mˆ2, obviously we're looking at fighting up to 46500-51000 lbs or 21136-23181 kg of force. I doubt the reality is anywhere near that high, since the THS trim range can
generally only be a few degrees out of alignment with the slipstream/airspeed - that's where the trigonometry fits in. Along with the other factors mentioned - altitude/air density, speed, pitch of the overall aircraft.
With which we can compare the available trimming force available (motor power or pilot muscle).