Nick, I'm not sure why you think that a loose TR cable would lead to a total loss of drive (the TR blades are stationary in the descent). If it were a broken or loose cable the ability to change the pitch of the TR would have been reduced or lost completely but the TR would still be driven. I don't know if this model had the same folding tail pylon as our military Wessex did but if so, the disconnect coupling would be a prime culprit - the profile looks exactly like a friend of mine had a few years ago in a SAR Wessex.
BTW Blender, the only civilian S58 I have seen did have collective mounted throttles.