Let me start by saying I have no more knowledge of this incident than is in the media.
The bottom line is, nobody has a clue what happened. There is no “stand out” theory that makes complete sense.
We are dealing with a part of the world ruled by military Juntas, where secrecy and face-saving are national pastimes.
As for the possibility of shadowing the Singapore Airlines jet – yes, it could be done, but for it to have been part of a plan, you would have to know exactly when and where the SA jet would be, and that information just isn’t available to a pilot in the cockpit.
Information released so far seems to suggest a hijack, by either one of the pilots, or a passenger with high levels of aviation knowledge. My best guess is that it was either shot down(military trying to avoid 9/11 style attack) or that it tried to fly somewhere without enough fuel (Somalia?), and crashed at sea. This is pure speculation though.