@caper:
I would have see this in ink. I believe that heading could be lost. However, NAV data is saved somewhere else. HSD, STPT and NAV carrett should fuction with just gps in ATT mode.
I have it in ink on my CDCs that the USAF issued to me as an avionics maintenance technician. I am not going to go uploading those to straighten you out on this or any other question here though. You will not have any steering data with a dead INS or even with a perfectly serviceable INS that was placed into ATT mode. All of those things you assume you will have with only GPS are wrong.
If you happen to have or see the legit 34-1-1 that is floating out there page 1-153 first paragraph refutes all you state to be true above.
@caper:
ATT mode is not spiket that limits the flow of INS info to fix a broken INS.
Not even a clue what you are attempting to say there.