<... that there was no longer any indication of how accurate GPS was ...>>
I believe that was only the case when the Doppler system was not functioning (and I mean not functioning/working as opposed to not being selected) - as I believe, even with GPS mode selected, the system continually compared the GPS position with the Doppler position and flagged it up if the discrepancy was beyond a certain amount (makes sense, don't it?).
Even with GPS mode selected, I believe that for generating steering guidance the system still took several parameters from the Doppler side of the STANS - I'm travelling just now and so can't getback to my usual computer to dig out the stuff so this from memory just now - perhaps this input is not known about by those who use the system to navigate - even if you are in GPS mode (as is my understanding) and have STANS (or what ever the nomenclature is on the button - DGPS/TACNAV - I haven't seen an HC2 HoSI mode select panel close up) selected on your HoSI there is an influence from the Doppler system in what gets to the HoSI.
Don't forget, the GPS of the time, apart from the odd glitch and low update frequency (once a sec?), was lagging about 2 seconds.
I hope 240 was not trying to say that STANS as a whole was not selectable to feed the HoSI! - if that were the case, I'd have to agree with one of the popular mantras "the cause of this crash will never be known" - how the HC2 was navigated would never be known!!!
How about talking this aspect through? - getting that overall systems understanding?
Please Tuc et al, don't deflect straight away with airworthiness and interference etc - there is clear need to focus on the basic nav just now - or no one will be able to follow the arguments.