The odds of a pilot having a heart attack just when the other pilot's in the john are pretty slim. Possible, but pretty unlikely.
And you can bet, an autopsy is not possible, due to not enough of the pilot’s body left.
The other pilot wouldn't leave cockpit until the airplane is at the cruise altitude and on autopilot. At that point there isn't much to do. If the pilot in the chair suddenly got incapacitated, why would he be disengaging autopilot and then carefully executing the descent? The autopilot would guide the airplane through the whole course, and the crew outside would have at least an hour or two to figure out what to do. We say many things about Airbuses, but I doubt that any autopilot these days will just reach the end of programmed path and turn itself off, letting the airplane to fall to the ground. There were crashes that involved dead crew (such as in Greece,) and the autopilot kept the airplane flying until all the fuel was used up.