Duncan's prefatory remarks below.

I will say up front that I am not a fan of Duncan Steel. Quite the opposite. However, his concern relative to a "conceptual error" misses the mark. I have provided a steady stream of criticism to the IG relative to the constrained AP flight dynamic assumption. While the IG is loathe to make any assumption relative to motive or causality, they seem quite ready to accept the constrained AP flight dynamic as a fact. Very strange.
I have a very high regard for the IG relative to their analytical skills. The latest retreat they have made, again very well executed, is not going to get it done. They are basically tweaking old ideas. They continue to use the same tired assumptions, and tweak them to the North. The error is a not a tweak away from being correctable. The entire IG conceptual framework is fundamentally flawed. The ISAT data is not a predictive tool. It is a qualifying tool. It can be use to eliminate the Maldives and Bay of Bengal. It cannot be use to predict where the aircraft terminated on the 7th arc.