[Spirator]

(1) What you may discover is that yes, PG71's will backdrive on their own, but connected to a lead screw they shouldn't. Of course, this all comes down to the pitch of the screw. I don't know the math, but it's unlikely that a vertical load will cause the screw to undo.

(2) You can set the Talons to brake mode and they'll effectively 'short' the motor terminals when no input is given to the Talon. This will result in back EMF which will resist the rotation of the shaft, but I doubt it'll completely stop it from happening.

Best of luck!