3) Why did the helicopter ground speed drop to near 0 at the end? What events led to that?
4) Why did we lose the signal at such a high altitude? Earlier in the flight, less than a couple of miles away, ADS-B exchange received the signal as low as 775'.
Those questions track with my thoughts....and your comment about unusual aircraft movements confusing the Pitot Static system and data reporting based upon that all seem to suggest those might be correlated with a catastrophic failure of some kind.
Which leaves us without the bigger question of what kind of failure it was and why?