So, are we assuming that the 19:40 ping line was traversed in an easterly or westerly direction? Assuming the plane was travelling south east initially makes sense but then forces a much more southerly heading for the timing to be right for the 20:40 ping. Assuming the 19:40 ping occurred with the plane travelling west requires "the hook to the north" between 18:28 and 19:40 referred to in the Australian report but then allows a more plausible track through the 20:40 arc and results in a constant speed track which intersects the Northern end of the proposed search areas.
In the midst of all this, there is a glaring anomaly which doesn't go away unless and until you have accurate fuel figures. They have not been released
AFAIK.
When all is said and done, you cannot make a good enough prediction of the terminal position unless you have certainty about the direction of travel at 19:40 and the fuel load. Even then, you must indulge in some educated guesswork to narrow down the possible tracks.
In the report, we have some clues which I think are important. The report states the likelihood that the southerly track was flown on autopilot and therefore, I am inclined to believe that any solution which would require any heading or speed changes becomes highly unlikely. This in turn, implies (to me at least) that the 19:40 direction was west because that fits in with the 20:40 ping and a plausible speed and heading.