Okay, the more I think about this, the more loathe I am to think a navigator of Fred's training, experience, and ability wouldn't know the Phoenix Group was there. It would seem an incredible coincidence those islands would be on the 157/337 line without him realizing it. Suppose he had the same map the Navy had, and just as they did, just extended the lines and drew in the Phoenix Group, maybe even writing in latitude and longitude for them. He may not have even written the names in, as he would have considered it an absolutely last ditch option anyway. I could see him not sharing the information with A.E., as her lackadaisical approach to navigation and Morse Code would indicate she was moving more into the entertainment/fame business that serious aviation, and would probably not be interested. After the alleged head injury, he was unable to share any of this with A.E., and she would just give the coordinates she could find on the map.
Of course, none of this can be proved, but is just an exercise in thought. It may help in putting some pieces in place (or not).