I have prepared a more accurate map of the various constraints governing the possible southern destination.
The red circle shows points that see the INMARSAT IOR at 40 degrees altitude. The plane was on this circle at 8:11am. (It may have been on the northern arc of the red circle not shown but I am just considering the southern possibility here.)
The pink circle shows the limit of INMARSAT POR over the pacific. The plane was not east of the pink circle. Otherwise POR would have detected a ping at 8:11am as well.
The green circle to the far west is the limit of INMARSAT AOR-E over the Atlantic. The plane was not west of that line.
The light blue circle is roughly the limit of distance that the plane would have been able to reach by 8:11am given its last sighting NW of Penang at 2:15am. So the plane did not have time to get south of the blue circle.
(Assumes 5400km from Phuket)
The white circle is the limit of Australia's JORN radar. If the radar was switched on at 8:11am (WST=MYT) then the plane may have been spotted if it was within the white circle. (Assume 3000km from Laverton).