Three Miles
The only thing I am missing is:
in which AP lateral mode (mag track? mag hdg?) would the 777 have flown, if LNAV along waypoints was ruled out?
I think they have jumped to a conclusion that isn't justified. There is no reason why the aircraft couldn't have flown using LNAV to a waypoint or destination miles beyond its endurance. My theory stated many times is Wilkins Runway as destination and that is being backed up by the evidence in the report.