Were the pilots friends at all , had they flown together recently , it doesn't sound like a 2 crew spur of the moment Idea.
Hijack is possible , the unaccounted for ULD could have contained the perpetrators and weapons. It is possible to get from cargo to EE bay to cabin.
Could disable systems including cockpit door and walk right in.
Leaves 2 possibilities
1. They got away with the aircraft and it's hidden or on the sea floor.
2. The plot was discovered and the aircraft shot down and now covered up for some unknown reason.