With the first ordered motors in customs limbo I've ordered some alternative motors with brass nuts (which may be needed for higher forces). These will take a few weeks to come through, so I will see if i can put together some hand calculations for the forces and a finite element analysis of the fused nylon mechanism to see what the local stresses are and if there will be fatigue issues.

All of the motors look like they were originally for use in camera lenses. So one of the complications is working around the mounting arrangements.