So, the editor who worked on the 5th grade book in the Wonders program is no longer there, but the editor I spoke to told me that for that and a number of other stories that they commissioned (rather than licensed) the procedure was to do online research to find something like the kind of story they wanted. Then they would hand over what they found to a writer. The story I told you about was written "to spec." So I'm thinking that First People was likely to be the source--and it's very disappointing that First People didn't keep better track of their sources.
You should not be surprised that school curricula sources point back to the one First People source. Teachers are not academic researchers.
I have to tell you that there's a lot of quicksand on the Internet. The World Wide Web has now been around for 20 years, and over that time, rather than more and more original "content" coming onto it, either from books being scanned or stories or information being written new, there's a strong tendency for stuff to get reused over and over. Even now, there's a lot of print material that is not available on the Internet.
It's entirely possible that this story, even taking into account Stephanie's version, could go back no farther than the 19th century, when it was published in a newspaper or magazine. From there, it passed into the oral tradition, got retold in Stephanie's family, and in other families, and got back into print.
And that is why as a general rule when researching folktales you need to have multiple sources, at least one of them print, and confidence that they don't all go back to a piece of "fakelore" written in 1873 ... (I know you know this. I'm saying it as a general point to others working on folktales).
By the way, have you come across Aaron Shepard's website? He's a picture book folktale writer and he has information about the process on his site, or at least he used to.
Good luck! Sorry I couldn't be more help.