mm43, I am not sure from reading your scenario and from the picture that you factored in the mass and viscosity of the water. It takes time for the water to get out of the way. I rather envision the plane was somewhat flattened by the time you have the plane in the trough it makes in the water. That would violently compress the air in the cabin adding to the shredding effect on the body of the plane.
Water hit at 50 MPH is not too unlike hitting a wall. It does get out of the way. But that takes enough time to break things that hit at that speed, especially with a er ah "belly flop."
That said, I'm with you in regards to the impact being enough to cause the pelvis and back injuries cited. I'd expect it to have hit closer to 100MPH vertical and horizontal both. But I suspect BEA has the engineers and data to make the reverse calculations leading to their estimates.