If the fin had parted at altitude, its assumed aspect after "stabilizing" would be LE down, trailing the Rudder.
Bearfoil,
have you ever seen a video of a wing coming down after separation ?
In all cases I have seen the wing was tumbling down in a flat attitude.
Never I have seen a wing coming down LE first.
That's due to the Lift being created by the LE.
Center of Lift on a Wing is roughly Quarter Chord. CG is roughly MId- Chord. So it is longitudinally highly instable.
That's why in a flying wing plane you have specific S- shaped chords and the CG far in front of the Quarter Chord line.
Try flying with a wing only where the CG is mid Chord: Good Luck !!
So once the Wing would start falling LE first and would build sufficient Airspeed the LE would rise, thereby flattening the attitude, likely starting a rotation.
And that's exactly the behaviour you see in VidCaps where a plane lost its Wing. There was an ugly video on ysoutube of a P-68 shedding its Wing during aerobatics.
The forward/bottom corner would enter the water first, explaining the lack of further LE damage upward as the fin tapers in chord and width.
Esactly that would NOT happen !
see my explanation above.
It would hit randomly, possibly rotating but generally flat.
And that is what makes the upward bending of the lower ribs in the VS perfectly fit to the assumption of BEA that it got lost upon impact in a downward direction and not so much to a loss at altitude which would probably be more sideward than anything else.
Having said that I wouldn'i rule it out completely but I strongly tend to agree with BEA on this particular aspect.