I have reservations about a flat spin with no forward motion; but with the information publicly available it may be impossible to rule it out.
The theory of a flat spin is completly unfounded. I don't know where this idea started, but I guess it came from Bea's expression "with a high rate vertical acceleration".
Now think just one moment: If you would flat spin a widebody airliner (which I still believe is impossible), what acceleration happens during impact? In the spinning axis of the fuselage, right in the middle, there is nearly zero forward motion and heavy negative vertical acceleration. Perfect for the flat spin theory.
Now change to other parts of the aircraft, e.g. the tail. What acceleration appear there? Because of the rotational speed, there would be heavy decceleration to the side. Which is known to be not happened. Because the vertical stabilizer was found as a whole. It would have simply been clipped of the fuselage.
That's what Bea wanted to tell us: All forces they could assess where vertical decelleration. This happens in a relatively straight flight profile with high sink rate. There was some forward motion, too, I'm sure. But the forward decceleration was way slower and thus didn't deliver marks on the wreckage.
So, please, abandon the spin theory. It's useless.
Dani