Too complicated.
My initial guess was heart attack, and I’m sticking with it.
Stick right, and then foreward, which would be quite likely if it was his tikker, would produce the observed motion.
The observed motion was a sudden gain in altitude, then a roll over, then a dive into the ground. That motion is not consistent with your theory, but it is consistent with the theory that that pilot seat broke loose, and when the pilot went backwards he brought the stick with him causing the plane to climb suddenly, by going backward he also lost rudder control causing the plane to roll over, because he had the stick held back when the plane rolled over the climb turned into a dive.