Old Engineer
I for one am immensely satisfied with your reasons for VS failure. I caution others not to rely too heavily on instinct, (though I may have!) It is invaluable if used appropriately. I found my inability to complete the picture frustrating, it is a bit embarrassing I took what may have appeared to be an "All or Nothing" position. I consider it less likely the assembly separated at altitude.
There is a caveat, naturally. Although the water impact is easier to "picture" as of now, there are two things. One, the Arm36 tip may have departed at altitude, if the Rudder was subject to emphatic Torsional Reversals, (perhaps even flutter). Two, to ennable the great load required to "flyaway" the VS, (diminished somewhat by your work),
Hazelnut's figures and intuition remain, and I believe the sudden Horizontal cease of motion was ennabled by the sinking of the Nose, at the end of the "S" curve I propose is the profile of the CG from tail impact to cessation of motion (absent the final buoyancy sequence).
Let's not forget that the tip may not have been present on the a/c at Take-off. A night Walkaround would have spotted it, perhaps, then again...
bear