I don't think the fact it was the southern arc implies that it must have been an act of terrorism. After all, flying over the southern arc would put them more closely in line with having to land in countries like Australia who would not be sympathetic/accommodating whereas in the northern arc there are more countries where terrorists might be able to hide more easily.
It's true that to have flown over the northern arc would have made the plane more detectable though, but why (and the implication seems to be that this was the pilot not other sources) remains to be seen.
I also don't think that there has been more known than has been released, unless they could have been sure that the plane had gone down with no hope of survivers. At no point has terrorism been muted as a possibility, and especially now the americans are involved this would be something which would have been acted on very quickly, even as a precautionary measure and it hasn't.