Aircraft lore is made of these kind of questions!
For all S-76 models:
1) The tail rotor is driven by the main bull gear, and thus by either engine.
2) It is not possible to isolate the TR drive without postulating a failure somewhere.
3) For convenience, the TR drive on the S-76 family MGB is twinned from the #1 side, of the input drive assemble, but once again, the #2 engine drives the TR through that gear, as well.
Regarding the TR overspeeding, the engine overspeed should catch a runaway before the TR gets to 120%, and the TR has been tested many times to well above that, I have personally seen 125% Nr/NrTR in flight.