....”Which raises questions about which military or civil authorities, acting with or without the official sanction of their governments might be involved, should the aircraft have been successfully diverted?”......
Which, (though I haven’t followed this closely), would definitely be my thoughts if indeed it has landed somewhere....and what country would gain by holding 239 people hostage the most...and where would they put them if it landed? Assuming it did land...and not crashed somehwere.
Hostages/plane for ransom.
Hostages for technical expertise or other knowledge (forced labor/interrogation).
Plane to re--purpose for terrorist or other (illegal) use.
Something in the cargo hold we don't know about.
This assumes a high probability of recovering the aircraft, occupants, and/or cargo intact.
A terrorist act would generally rely on the destruction of the aircraft, the occupants or contents, and possibly objects on the ground or at sea. (Find a naval group, declare an in-flight emergency and intention to ditch the aircraft, and go Kamikaze, for instance, or something more on the order of 9/11).
The latter was not done, as far as we know, so the intent may be to recover the aircraft and/or contents, successful or not.
That does not preclude later use as a terrorist device, and the wait, a paint job, and reprogramming to spoof towers into believing the aircraft was another plane would take time, time which would get people to let their guard down as they begin to assume the plane is lost.
By leaving the question open of whether or not the aircraft, passengers, and contents survived, eventually the assumption may well be that they were lost and not recovered.
For someone after an aircraft to re-purpose (assumed lost), or the knowledge of those on board (assumed lost), or the contents of the cargo hold (also assumed lost), it would be far, far better to let the world think the plane had gone down--no claims of "responsibility", no ransom.
Which would mean occupants not found to be useful would likely not survive very long.