It was going fine until you said this:
an immediate yaw leading to loss of lift and low G resulting in the rotor being unloaded and the head hitting the mast and bending the mast at the transmission, ripping the entire lot out of the roof
But, I’m not opposed to the idea that the transmission detaching could have been caused by a combination of dynamic, and aerodynamic forces, resulting from the loss of the tail boom just aft of the mounting point. Why it detached, I have no idea, but I very much doubt it had anything to do with a flailing TR drive shaft. Because of the speed they run at, they experience low torque and so are very lightweight. My earlier remarks were connected to the suggestion of the pax applying pedal.