I actually suggested something somewhat similar a long time ago, only my theory didn’t involve AWACS or shoot downs.
As a general rule, the more people involved, and more elaborate a conspiracy, the least likely it is to succeed and stay a secret.
It has been consistently rumoured that someone onboard was carrying some very sensitive technology destined for China.
That’s not the sort of thing you put in your check in luggage, and it would have been almost certain that Chinese agents would have been accompanying it as close protection.
How do you steal such a package without bloodshed or causing a major international incident? The easiest way to do it would be to introduce sleeping gas in the airlines energy air supply. If the US was desperately enough to risk intercepting a civilian airliner over international airspace, they would have just deployed agents to grab the package before it even got to the airport, with a massive shootout if needs be, as that would be less of deal that such an intercept/shoot down.
For my idea, all you would need are the pilots, and someone with ground crew access.
Pilots could be CIA assets with shallow covers, and can be switched in at the last minute without anyone really raising any eyebrows since such things happen all the time in civil aviation. Likewise it would be child’s play for the likes of the CIA to issue fake passes and credentials to allow one of their people to gain access to the plane as ground maintenance personnel. All very easily arranged.
The pilots would put on some aerobatics and drop emergency oxygen masks and order everyone to wear them, which everyone would automatically comply. Sleeping gas goes to work and knocks everyone, including the canine crew out, co-pilot pops out and casually lifts package and locks it away safely in the cockpit. Pilot shuts off sleeping gas, everyone wakes up, and mass unconsciousness will be chalked off to cabin pressure loss. Story makes the news as a minor side note and pilots gets commendations for ‘saving’ the plane and passengers, but decide to retire early due to the stress of the incident.
Problem was someone messed up the gas mix and no one woke up.
Pilots freaked out after realising they were flying a plane full of corpses towards Chinese airspace, with irrefutable evidence they cannot get rid of onboard that directly implicates the US government in a mass murder. They called their bosses and were ordered to drop blow radar and head to an US airbase, probably on Diego Garcia.
AWACS may have been subsequently scrambled/diverted to provide tracking and navigational support to ensure no-one picked up the plane thereafter. Maybe some jamming was also selectively used where the plane could not entirely avoid civilian radar grids.
Incidentally, this disappearance is rumoured to be one of the main reasons for China’s decision to build its SCS islands, as those islands now provide radar coverage over a previous blank spot, so things like this will never happen again.