I solved so many crimes over the years by carefully considering the psychological, human nature aspect of the people involved. For the sake of this discussion I'm calling the series of radio transmissions the "event" and Betty's reporting of the event the "report." As an investigator for each data point I would ask myself - "does this make sense? does it feel right?"
1) did this event really happen or is Betty lying?
2) if it happened, was it from AE or an imposter?
So straight away I would try to disqualify Betty by asking "does it make sense that a young girl would be using a shortwave radio as she claims? Seems kind of "techie" and not what a girl would do...well not the girls of today. However I'm 57 so I remember the days of only 3 tv channels which signed off at midnight with the Star Spangled Banner. Radio back then was the main pastime - people got the news and listened to music & people like Betty were "star struck". Dads all over America strung antennas for their radios. It feels right. And Betty records her routine pastime activity in a contemporaneous notebook. Her drawings and musings feel right. Her alibi of being home in front of the radio passes the test.
And see how neatly her personal life weaves in and out of the "event" in her notebook? Here she is retelling the story of not only her life but this second story of AE's life and the two are remarkably woven together, preserved forever in her precious notebook. Who fakes that? Then there's the event with this "open mike" where Betty is able to record the emotions and poignant struggle for survival that transpired in the cockpit in addition to words that were transmitted as a formal call for help. AE fades away and Betty goes back to her personal star search until the material weaves its way back around and snippets of AE come through again. Betty is our human CVR. She couldn't be making this up, she really heard the event.
That's some of the psychological side of it. Then the actual words in the event, although circumstantial evidence, really help to solidify the validity of the report, that Betty heard it and AE said it, not some fraudster. "Get the suitcase in California" and the similar wording in a letter known to have been penned by AE, "New York City (sounds like) "and the eerily similar Norwhich City. What little girl or fraudster would think "hey, Norwhich City ran aground on this little atoll, let's pretend AE landed there and let's give 'em a hint but not too obvious now so say New York City"??
What about the shortwave radio? TIGHAR has tons of information explaining how it works. I have empirical evidence. When I was a young patrol officer I was assigned the southwest side of Austin, David Sector. On the midnight shift our Motorola radio David frequency was notorious for radio skip interference coming from gulf coast shrimp trawlers hundreds of miles away in Port Aransas and Port O'Connor. There would be routine police traffic on the radio then a blast of expletives followed by beer soaked invectives. Around 5am the atmosphere would change and the skips would cease. So it is possible for AE's transmissions to make it to Florida. It feels right. With the way harmonics act, and all the different brands of radios, different brands and lengths of antenna wires, the various skill levels of the installers and the magnitude of the local radio interference I would not expect any two radios in America or the world to hear exactly the same thing that AE said.
Now, top that off with "I'm traveling on the line" that's runs through Gardner & multiple direction finders pointing to Gardner...please. It's incomprehensible to me that either Betty or anyone else faked this.
And Ric I just want to say I am extremely impressed with your exhaustive depth of analysis on recovered artifacts, & the professional manner of your investigations. Extremely impressed.