Deniability would have been a very likely outcome if a convincing crash scenario had been engineered. I suspect that the pilot was supposed to turn off the satcom link at the same time as the transponder. Now that we know that it kept flying for 7 hours and may have been shadowing this other plane, it looks clearly like deliberate theft.

If the last contact of any sort had been the 'good night message', then we would never have known.

Totally unnecessary and melodramatic, in this hypothetical situation all you need is a plane that has no attachment to the nation in question.

Imagine I wanted to kill a person, and I've decided to use a car to do so. I can't use my own car for obvious reasons, so any stolen car will do as long as it doesn't connect to me.