I'm not a pilot and I apologize for intruding in a professional pilot forum.
Just thinking "Occam's razor" ...
Could this be just a case of a botched stall recovery? We don't know what could have cause a stall (pilot's error, unreliable IAS reading due to obstructed pitot tube, weather). But then, the pilot didn't apply proper stall recovery technique, resulting in loss of both speed and altitude, and by the time he pushed the nose down to regain speed it was too late (not enough altitude) and basically flew the airplane into the water. This would be consistent with the GS and ALT plot, wouldn't it?