Rotorfan-
"I notice that not all helis have a vertical stab, so all yaw-control is by tailrotor only."
Sorry, can't go with that. It is just a combined V and H stab. It still helps with
directional stability about the yaw axis and also holds the tail down as you would expect a H stab. to do on may types. In that respect it is aiding stability about the pitch axis.
If that were not the case and the Heli suffered a COMPLETE loss of TR thrust then nothing, even entering auto would be likely to save it and the pilot.
First important thing on TR 'failure' (in my opinion) is that many of the occurrences heaped under this banner are completely misdescribed. If it still works but you lose control of its pitch (for whatever reason)then it is not a TR failure at all, but a TR control failure.
If the driveshaft breaks it also is not TR failure becuase you still have a viable TR but nothing is connected to rotate it. It is a TR drive failure.
Only if the blades become detached (for whatever reason) is it a true TR failure.
I know that Helimutt is poised and well able to describe the techniques used in the various situations above and I'll move over for him to describe them.