Given the press conference and the SAR grid update, are we now able to assume that an 'event' causing it to crash into the sea at IGARI is now not the favoured theory, unless it slipped under the surface at this location without trace, although there is some precedent for this it does seem unlikely.
Therefore it flew for some distance after the last piece of public available data and crashed somewhere else. I'm discounting the theory that this bird is somewhere on the ground intact. Assuming the authorities have more detail than us (very much hope so) the change in SAR grid does seem to indicate the plane flew for at least another 30-60mins before crashing somewhere.
That leaves us with the option of an unlikely catastrophic electrical failure at FL350 causing the transponder to fail along with anything but basic instruments & controls, perhaps a RAT failure so no chance of radio contact. Then an attempted return under VFR at night, got lost, fuel starvation and crash. We then need to factor in the failure/non activation of the ELT.
In other words the last two paragraphs need a lot of holes to line up to occur.
The more realistic option based on this (sad to say) is someone pulled the CBs in an attempted hijack, flew for sometime, passengers/crew attempted to retake and the result was a crash.
In both cases the SAR area is much larger as the plane could have flow in any direction for 4/5 hours. I think it will be found but outside even the current search grid.