First time poster, long time lurker here!
Hopefully I can answer some of the questions posted about the TRA (Tail Rotor Actuator).
The input lever will drive a dual main control valve to control both hydraulic systems. The position feedback is via the trunnion block (90). I would imagine, and I don’t know this specific actuator, that the control valve will be spring biased to return the actuator to a neutral position if the pilot input is lost. If the position feedback is lost the actuator will run away to a hardover position.
If the washer (140) was incorrectly installed position control would still be present, but the null and full stroke positions of the actuator would be affected . This will be installed and checked (visually and by test) at the manufacturer, as would the nut (60) and cotter pin (50). These parts would never be removed from the actuator unless it was undergoing some repair work (removed from the airframe).
As far as a nut going missing, I think that the thread is handed such that the rotation of the Tail Rotor always tightens the rod into nut. If the nut is missing then I would guess that it was a mechanical failure somewhere.