Yes, the diameter of the manual trim wheels changed and, yes, it was a long time ago. It's possible that there is some side loading of the jackscrew, but probably not enough to be a major factor, considering the angle it forms with the stab lever arm and the fact that the ball nut is in a gimbal ring. Yes, pilots are no longer taught the porpoise maneuver, but it has no chance of saving the day unless there's sufficient altitude AGL.
Almost certainly, the real problem with manual trimming in the sorts of situations we are considering is just that the H-stab is huge and, at some combinations of attitude, (out-of-) trim positions and airspeed, the aero loads make it either unlikely or impossible to crank those wheels enough to recover.