I think the known course change, and its direction to the closest long runway airport, means there was a major event on-board.
Could it have been hijackers?
The turn to the airport wouldn't suggest this for two reasons,
1. hijackers would have their pick of 360deg course change or no change at all(probably best bet); to pick the heading for the closest long runway airport would be unlikely but not 0 probability.
2. if hijack was underway but cabin was still secure, the course change would be understandable but no radio call after that wouldn't be
And then the 3rd reason of a no hijack would be if the hijackers did take control and shutdown data/radio, why no other course change and why no cell phone traffic while over land.
The hijacker(s) would have had 45 minutes to disable all passengers and crew in the passenger area after take-off, but I would think there would have to have been alot of hijackers to do 200+ people fast enough so that no calls or text were sent.
Maybe a gas was used..........really a stretch on that, I would image someone would notice a gas mask, lol.
Maybe just one or two hijackers, they go into the bathrooms and put gas masks on, release the gas and wait a few minutes, Hollywood style.
Not sure you could do that without raising alarms in the people effected last, people will be walking in the aisle when they collapse, others would notice that, at least to say "Hey, I want some FREE drinks too!"
Suicide, again we have the course change, why?
I guess the mind is a bit off to do the suicide thing, so we can't really decide by logic what someone would or wouldn't do, that's like trying to get in the mind of a school shooter, that forest is just too dark to see any reasons for "why?", at least any reason that would make any sense to us.
So pilot or co-pilot suicide is not off the table but would be a very very long shot indeed.
Terrorist/suicide, no, for sure not, that dog can't hunt in a run out of gas crash, so far away from point of doing it as to not be remotely plausible.
And "no Virgins for you", autopilot did the suicide..............
So no, doesn't make any sense.
My guess is still the lack of oxygen/CO theory, but possibly caused by an on-board fire now, effected passenger area first and then flight crew, when crew realized what was happening they headed for closest long runway airport, fire was contained but caused electrical issue in data/radio and crew succumb to lack of oxygen and CO shortly after course change.
No use reinventing the wheel on this, flights that have ended this way, running out of fuel without course changes, are pretty much caused by the same thing, everyone on board is incapacitated, autopilot was on.