Hunter,
While it could be anywhere on the red arc at 0811, Inmarsat sensibly curtailed the arc at a spot in the Indian Ocean that is the number of flight hours multiplied by a reasonable speed from the South China Sea. They presumably have several earlier hours of angle measurements too, which would be consistent with that.
It seems unlikely to me that the Australian government would make such a big deal about going to look in that oceanic box unless they had some good information that looking in that box was likely to pay off.
The biggest problem with the northern route is that someone somewhere should have seen it co,ing, whereas going south there's nothing to bump into.
The earlier Inmarsat ping angles would show whether or not a steady straight flight into the void was consistent with them. I trust the heavy Australian search is based on sound information, although their released pictures of the "ocean objects" are very whitecap-looking to me. The radar shenanigans and turns over SE Asia all seems to be pretty random.