Would you notice a loss of TR thrust at 140 kts in the cruise? The TR can't be at that high a pitch with all the tail boom and vertical fin working for you.
Crab, I was told long ago that if the TR stops rotating (as opposed to a TRC failure where it goes flat pitch but keeps turning) that you lose the same amount of weathercock stability as if you had lost a chunk of your vertical stabilizer of equal area to the TR area. I have no idea if that's true, perhaps someone else here knows for sure.
On the subject of controllability if the TR gearbox comes off, I was talking with Pat Cox at Robinson a year or two ago about the fact that I seemed to remember the R22 manual originally calling for a running landing for TR failure, and that I think it changed to calling for an auto in the 90's sometime. He said that they believe the R22 is uncontrollable (due to CG shift) if the gearbox departs, but that the R44 and R66 are probably controllable. My only point is that it's probably highly aircraft specific which ones are controllable if you lose the gearbox. I guess it also helps if you're not at the forward CG limit when it happens
