If the cockpit door is locked, it's locked. Nobody is forcing their way in.
Has it been verified that the plane dropped 40k feet quite rapidly? Could have been defensive move by pilots, or could have been a deliberate attempt to drop under most radar coverage. But if they were flying that low for that long over land, a cell tower somewhere would have eventually picked them up. I just can't see a jet like that traveling NW through all those countries' airspace undetected. Gotta be going the other direction into the Indian Ocean, but why? Crazy pilot commits suicide has still gotta be most likely scenario.