Sultan,
You bring up a good point with regards to taking a closer look at the effects the tail rotor drive failure had. The TSB report notes that the data recorder showed an indication of rotor brake engagement just a few seconds before the data acquisition ended. And I believe the rotor brake is located where the tail rotor drive exits the MRGB housing. The investigators seemed to think that excessive motion of the tail rotor gear shaft as it failed may have affected the rotor brake.
The investigation clearly showed that the tail rotor drive gear failed structurally. Under a loss of lube event, this should not be unexpected for a couple reasons.
First, the tail rotor drive gear was likely a spiral bevel gear. Spiral bevel gear meshes generate lots of heat and thus are prone to scoring. A loss of oil cooling to this spiral bevel mesh would quickly result in contact scoring, overheating, loss of mechanical strength in the gear teeth, and finally structural failure of the gear teeth.
Second, the tail rotor driven gear was supported by a tapered roller bearing set. Once again, tapered roller bearings tend to generate lots of friction heat where the roller big ends rub against the race shoulder. A loss of oil cooling to this area would quickly result in overheating of the rollers and race, loss of mechanical strength, and finally structural failure. These problems would be made worse if the tail rotor drive was subjected to high torque loads in addition to the loss of oil flow.
Just one more interesting aspect in the chain of events that caused this tragedy.