I think the assumptions made by the investigating team are only provably false if you have certainty about some of the variables and in particular, the point at which the SSE course was assumed. Once you have fixed this point, speed and heading are covariant and the range of terminal points becomes constrained in exactly the way you describe. If you take away the certainty about the starting point, and I think we must, the range of available terminal points matches those assumed by the investigation team.
The message is clear - information about the starting point is not available to the investigators. I believe that should make us circumspect about accusing the investigation team of incompetence.