Flipster,
I agree with most of what you say, but if you are saying that Waypoint change shows the Situation Awareness was not compromised at that point, you may be wrong.
Mr Cable confirmed that the TANS was in the Off position when he examined the wreckage.
Crashes change many things (including altimeter sub - scale settings).
I have used several varieties of TANS. They all require the toggle switch to be pulled from a detente ON position, then turned, then lifted into the OFF position. I can't see how any crash deceleration could do this.
Since the Waypoints make sense for the planned route, we can assume that the kit was switched on and it was intended to be used for the whole route.
The only reason the crew would swich it off is if they thought it was feeding false information. Lack of confidence in the kit can be caused by equipment problems or perhaps because of false perception of position by the crew?
Does anyboby know if the TANS was swiched off before or after Waypoint change?
Even in the event of loss of GPS signal. The TANS in Doppler and ADS modes is still an improvement on HAT.