Ditching under control
While I have great difficulty understanding just what is to be gained by mysteriously disappearing without a trace, there's a possibility that VNAV could be set up to start a descent at a waypoint where you would be down to minimum fuel, but you would still have to be conscious to put down flaps to achieve a ditching at minimum speed.
Without flaps, the speed would be much higher and we could expect some breakup and release of buoyant items in the fuselage.
The longer we go without turning up any debris, the more it looks like a controlled ditching - - with the very faint possibility the Inmarsat folks missed something in their calculations.
Given the more Northerly position estimate of the ditching, it's reasonable to expect some debris, if there was any, to make its way to WA. The drift models will suggest a time when it may be useful to check WA beaches at low tide.