Most curious (and frustrating)! I'm sorry that adjustment of the magnets proved only to be an elaborate shell game. I wish that I had some other suggestions for you, but the only other common cause of trapezoidal distortion in a small monochrome display -- yoke tolerances -- is effectively precluded by your not having noticed the problem earlier.

About the only other thing I could suggest is to experiment a bit with moving the yoke connecting cables around. Those carry substantial current, and it's barely possible that the associated magnetic fields could be influencing raster geometry by this tiny bit. Dressing the wires away from the base of the crt would minimize this influence, so if they're presently near the crt gun assembly, perhaps this could help.

But it's a long shot at best.