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. Some mode where the plane spiraled in, nose up, seems a bit more consistent with what the BEA is really describing. At a rough guess (very rough, but the best possible with the public data) the plane probably needed about equal forward and vertical speed, say 100-150 MPH each way. And a small bit of leftward force at impact.
The BEA analysis of the tail separation and the "vertical acceration" they describe for the other damage seems to be at odds. The VS needs a strong force to the front of the hull, yet everything else is described as having a force downwards. But this might not be a contradiction if the plane came in at 45 degrees forward and down, and hit somewhat tail first. (Flight angle 45 degrees downward; pitch maybe 5-20 degrees nose up.)
Breaking the tailcone from HS impact with the water could pull loose the VS. This could also account for the damage to the bottom of the rudder. The remainder of the body slapping into the water and being arrested by the wings, combined with the forward motion, would provide the moment to pull loose the VS in a forward direction. Spiraling or turning moment to the right will throw the VS over the left side, as described. Slapping the body into the water will provide the "vertical acceleration" for the other damaged items, but there should also be evidence of some forward acceleration as the forward motion is arrested.
And possibly there is. If you carefully examine the picture of the galley tray rack, you will see that many of the right rails are bent down, but not the corresponding left rails. This implies that the trays were thrown to the right side and not straight down, or both rails would be damaged equally. (The bottom two rails are damaged on both sides, but that is understandable as things piled up.)
Question: where was this galley unit located? Is there any indication that the right of the tray rack would have been forward?