Aircraft was on auto at FL120 ahead were two storm cells
With a CB, sure. But there was no convective wx at the time. Not a spot on the radar.
Maybe there is a big rock under the aircraft which protruded into the cabin
I thought that too but the sideways view shows an empty cockpit. The same pic shows the roof curved upwards, and pretty well stretched, with the supports ripped off.
It's possible it was just the impact, at say -10000fpm (100kt) VS, but I don't quite get the physics which would eject seats and occupants in different directions, with the roof intact and in place afterwards.
When pilot gets speed under control, his ground speed is zero
Sure, but where would this come from? The MSLP chart shows nothing significant. You are looking at a 100-150kt wind.
This is the most bizzare accident I have ever come across.