Few remarks come to mind
Apparently those 3 well trained pilots did not manage to recover from a high altitude stall with a fully functional and structurally sound aircraft. They even recovered reliable speed measurements about half way in their descent but still ended up in the water. This is really puzzling and I believe we might still be missing an important piece of the equation.
For the fairly high tech measurements and recording equipments on board it seems that there is in fact not that much information being saved. I would venture to say that with modern FBW planes we should have all inputs covered as well as some "trace / log" of what the software is actually doing.
Although rare pitot icing is not unheard of. As other have mentioned it would be desirable to add some different airspeed measurement devices, as FWB can really become confusing with unreliable airspeed.