Trout - Flying at 45,000 feet wouldn't cause hypoxia. It would possibly put a strain on the aircraft, but unlikely to be serious. Flying at 5,000 is a radar evasion technique. Yes, people could leave the plane at 5,000 feet.
Multiple, sudden altitude changes would seem to point towards a struggle on the flight deck. Protracted flying at 5,000 indicates an experienced pilot who knows what they're doing.
Could have been a struggle followed by successful execution of 'the plan', whatever that was. Altitude data is still being analysed, as far as we know.