The history tells that when something so mysterious happens it's a normal tendency to try to find to solve it with complicated or even sci-fi solutions,( let's see Amelia Hearhart disappearance in 1937, it was even told that she could have been captured by the Japanese soldiers and kept prisoner, but what probably happened was just a crash in the sea because the airplane run out of fuel). So what I am thinking is that also in this case it could be more simple then what the whole world is wondering. Electrical failure, maybe fire, ACARS and Transponder failes, maybe smoke or an Hypoxia situation recognised by the crew which immediately tried to turn back to KL, tried to insert new data in the FMS ( under hypoxia you think you are doing right when in reality you do crazy things)and this could explain the erratic behaviour of the plane and then it just flew until it run out of fuel.
( by the way, I'm not familiar with the ACARS system, is there any alarm that the crew will receive in case it fails suddenly on its own? can the crew notice it in case it happens?)