Thanks, all. Sorry for the delayed "judging". I owe beverages to all who took the time to work this through - especially those who saw through my clever disguise, and took this to be MH370-related.
Part B of the problem is simple: can you relate your answer to the performance limit given in Figure 3, p.5 of the ATSB's June 26 report: "
MH370 - Definition of Underwater Search Areas"?
Background: the south-eastern border of regions S1/S2/S3 is confirmed to be the ATSB's original Inmarsat arc-constrained performance limit. This limit is defined as the line connecting the fuel exhaustion points of a series of constant speed paths, each beginning at the NW tip of Sumatra at roughly 1836 UTC, and each passing through the Inmarsat arcs at their appointed time (but otherwise as straight as possible). Maximum endurance - according to the ATSB, anyway - is thus achieved at the speed corresponding to the path which overflies the final Inmarsat arc by the
greatest proportion of its total length. This is almost - but not quite (must adjust for speed differentials) the point at which region S1/S2/S3 is at its
fattest, as measured in the direction of the generating flight paths.
(I have back-solved for the maximum endurance speed according to this method: it is roughly 430KTAS. I just hoped someone could corroborate and/or explain it. Does this put hawk37 in the lead?...or is the ATSB out to lunch?)