Avionista
It should be possible to calculate the turning moment about the HS pivot point for a range of ND trim units at various aircraft speeds and elevator positions. This should determine the range of forces which has to be resisted by the HS trim jackscrew. A ground test rig capable of applying this range of forces to a jackscrew could then be used to check the ability of the manual trim (both electric and crank wheel) to apply NU trim under all circumstances.
I sincerely hope that Boeing engineers are doing this as we speak, otherwise they won't have a leg to stand on if this ever comes to trial. Saying that it was designed and calculated to do X and Y back in 1968, is not going to impress a jury...