Copilot pops an artery in his brain, and in this confused state he selects a low altitude, for some reason that the dying region thought was rational. Warning buzzer sounds, and his brain has enough remaining capacity to recognise, and as a response to press cancel for the alarm. Not enough left to fully respond, and as it is high brain failure breathing is unaffected by this, midbrain is still fine and controlling the body. Autopilot is doing the job and doing a descent to 100ft, and it is a very well known thing that the autopilot is not capable of doing terrain following, as the typical usage is in free airspace in a defined flight lane, it will use collision avoidance as an emergency for planes equipped with the system to avoid other aircraft. Approaching set altitude and rising terrain, it does a "controlled descent into terrain", though at flight speed very little survives.
Having the autopilot not allow descent unless at an airport is suicide, as often airport ILS beacons are off for a fault, or they are not present at smaller airports the plane is capable of landing at. Adding the requirement will make the autopilot very brittle design wise, as then any failure of the nav system, or any nav error ( GPS loses lock in heavy storms, and lightning can overwhelm the receiver so it loses lock until it does a full reacquire) means the plane cannot land, and then will fly till it runs out of fuel and then crashes. Do you want to be in the area of the airport approach when a plane in the holding pattern can run out of fuel in addition to a nav fault, and fly into the ground with the cockpit having the following repeating warnings " Fuel, Pull Up, Terrain, Attitude" with the pilots last words being a curse on you and all your descendants.
Identifying the remains is typically done with use of dental records ( teeth and jaw survives the crash quite well, and often even survives the fire somewhat intact) or by personal effects ( sim/sd card/IMIE label of the phone that was in pocket, or the remains of the clothing or wallet and passport) and as a last resort using DNA.