It truly doesn't seem to jell. If a/c impacted "flat" with high velocity, how did the VS separate 'forward'. Likewise, if the VS failed at altitude (separated) to fail forward the a/c was travelling 'tail first'. I also don't get completely the 'Ligne de vol'. Do they mean that the heading of the a/c was in its planned direction? That would be coincidental without directional control. How could that have been determined? Similarly, if the VS separated 'on site' why no major dimpling, compression, or tearing?
Its condition still suggests an airborne failure, leaving the Fuselage to decelerate and perhaps 'flutter' to the sea before the major portion of the airframe.