Talkspike,
Although the tail pylon is not cantered, the TGB is, so the effects are similar. Regarding coupling, the mixing unit does have a mechanical Collective/Yaw function, but not a trim related airspeed one that you describe.
The RFM has the following preamble to the TR malfunctions section:
With loss of tail rotor thrust, it is impossible to remain in level flight. This failure is associated with a loss in drive and characterized by a sharp, right yaw of the aircraft that requires immediate entry into autorotation (if in forward flight) or landing (if in a hover). If thrust is lost in forward flight (60 KIAS or above) roll excursions up to 10°, pitch attitude changes up to 5° and heading changes up to 50° may be experienced during the transition to autorotation. Once established in autorotation the aircraft can be flown to a stable pitch and roll attitude with an acceptable level of sideslip