Nick I have difficulty understanding how a basic cable operated TR control system can suffer a fault such as you suggest. The system only comprises a quadrant to change the yaw pedal input from rod to cable at the front end and a similar arrangement at the end of the tail to convert it back to rod and route it up the pylon to the pitch change lever on the TR gearbox. From there it relies on a magill bearing to transmit the pitch change to the TR spider. The only divergent mode I can imagine is a sort of 'bounce' in the TR cables which would mean the tensioner has failed and in many years of the brit mil operating the Wessex, I have never heard of such a failure mode. It would take some fairly major and very rapid pitch change reversals ( more than an autopilot with limited authority could manage) to trash a fairly beefy TR driveshaft like that on the S58.
I don't know what caused the initial left yaw before the failure (turbulence, control input, power change, autopilot malfunction) but a divergent 2Hz oscillation would surely have produced more than one wiggle before drive train failure occurred.
It matters not anyway as the pilot did a very good job, my friend who had the TR failure over a lake in Wales had the same difficult job assessing a suitable height to chop the throttles and ended up going in backwards with low Nr.