GHH,
I am not familiar with the 169 directions of rotation, or left/right hand threads for that nut. but can anyone say for certainty what could happen if the bearing seizes? will it rotate that shaft and un-thread the nut with enough force to overcome the torque, and safety locks?
If you look back through the message thread there are a few pics of the arrangement. The locking in the pic implies a RH threaded nut and the directions of rotation would imply your theory could be correct.
Normally the servo piston is keyed in some way to stop this happening but you can't tell from the IPC drawing. Having a split/cotter pin and locking kind of implies that this is the only locking. If it was keyed you wouldn't go to that length.
Similar things have happened before but not through design issues.
Once the feedback link is disconnected the servo would invariably motor to the end of full travel and stay there.
Pic at #682 and IPC at #630