JB
Thank you once again for your most comprehensive, and pertinent post. All I would say is this: The RNS252 Supertans, and the accuracy of it's sensor inputs are crucial to this case, not just because the crew may, or may not have been relying on the system for navigation, (for the very reasons you have stated) but far more importantly, because the Air Marshalls relied on this system as a 'rudimentary flight data recorder' upon which to hang their case. Information from this system, was even 'blended' with the Boeing flight simulation to provide their modelling parameters.
The navigation system, and it's ability to provide accurate historical data,(for which it was never designed) is I would very respectfully suggest, absolutely vital to the Air Marshalls case.
I have always felt a little uncomfortable with the fact that, as far as I am aware, the only people to have interpreted the 'raw' information from this system were the people responsible for it's manufacture. It should be scrutinised with the very finest of 'toothcombs', lest new evidence were to suggest the information relied upon was less than accurate.
Regards