Crab, your diagram helps clarify something for me. I know it's from a 139 but I believe the design principle to be similar to a 169 (although the pin carrier at the control end is different).
If the duplex bearing seizes and the outer race spins in its housing, rather than the inner one (the latter being pre-loaded against the stepped end of the control shaft), there would be no dragging force tending to undo the nut. Is the outer race pre-loaded in it's housing by torquing up the screws on the outer, dome shaped cover?