I wonder if anyone is considering just one possible motive for an apparent controlled flight (however far-fetched it may seem) to nowhere of 3.5 hours or so? A symbolic gesture?
Without spelling it out might it not be a good idea to search the area along that heading where the fuel might have been expected to run out from the last reported radar sighting west of Peninsular Malaysia towards a certain revered destination? Such a compass-bearing is seen in most hotel rooms for example.
Interestingly a quick glance at the map suggests that such a route might avoid all places where radar coverage would have lit them up.
If rescue is still an even remote possibility time is of the essence and if RR engine records are truly indicating 5 hours flight time this is certainly worthy of investigation.