There were many conspiracy theories

[Wednesdays]

One, as you pointed out, was that Earhart was spying on the Japanese on behalf of the U.S. government. A lot of books all but came out and said it directly, I think one of them was The Search For Amelia Earhart (1966).

Another wild theory was that Earhart faked her disappearance, and was still alive in the USA under an alias in the 1960's. Amelia Earhart Lives (1970) was a big proponent of that theory.