crab, I am not sure that they have found all of the pieces yet. There was some discussion about some of the tail trotor drive shaft sections not having been accounted for a few days back. (They may have been, of course, at this point).
As to how hard water is: yeah, our SAR swimmer deployment limits were 10 feet and 10 kts GS.
Flexible:
From the diagrams many posts up, the Tail Rotor system on the Long Ranger "pushes" in its anti torque function. (In comparison, the Seahawks/Blackhawks I flew had a "pull" tail rotor system).
Would the loss of tail drive not induce a right rotation of the nose rather than the left rotation seen in the video?
Further that thought, if the tail drive was lost as the initiating event, would that not reduce the load on the tail boom (the push is removed) and thus leave something else as the reason for the forward folding (seen on the video) of the tail boom? Some force caused that material failure.
If a tail rotor blade failed, though, or if a pitch link failed, that would set up a vibration that would manifest along the tail boom down to the attach point ... but wouldn't that be at a higher pitch/frequency than the 1 per rev vibration being discussed (as discerned from the audio?)
While discounting nothing, I don't think that the failures you listed would manifest themselves in what was seen on the video...something else seems to have been the initiating event / failure.