I am not saying that the following is what happened. However, I would appreciate it if someone could tell me where it is inconsistent with the information we currently have.
Pilot flying waits until flight is far enough from shore that cell phones won't connect, and at a point where coms are switched between different ATCs. Pilot flying sends pilot not flying on an errand or otherwise gets him to leave the cockpit.
Once alone, locks door. Pulls CBs/turns off transponder. Turns off passenger's flight tracking ability. Possibly activates cell phone signal jammer.
Descends to 29500. Puts on mask. Vents aircraft. Turns west, following suspected course avoiding most radar. Waits.
Once passenger oxygen supply has run out, waits a bit longer. Descends to low altitude below radar, and make turn toward final destination.
Land. Possibly on disused runway from Vietnam war, possibly even on road or beach. It doesn't need to be a perfect landing, as there's no intention of using aircraft again. Only needs to be good enough to walk away from.
Have accomplice waiting with another aircraft or boat. Unload valuable cargo onto boat. Sail away, never to be heard from again.
IMHO, it makes as much sense as anything else I've heard.