The point that needs to be made here is that any assumption for what happened between 18:29 and 19:41 implies an assumption about whether the plane was travelling east or west when it intersected the 19:41 arc. In turn, this affects the starting point for the long southward track hypothesised to have been flown on auto-pilot. It should be obvious that any constant speed or constant track solution for the later arcs up to 00:19 is rotatable around the satellite location. In other words, the terminal point is very sensitive to assumptions about what happened between 18:29 and 19:41 and the point at which the southerly course was started. The ability to rotate the track around the satellite without disturbing the timings for the southerly leg means that an assumption about the position of either end of the track (the point where it turned south or the terminal location) constrains the possibilities at the other end.
BTW, I have plotted my own ping rings for this and agree with the Australian report that the early rings agree with the submitted Malaysian Radar track.