John, you are indeed beating a dead horse and your logic gets more and more obscure.

Yes, "Maggadee" is aurally similar to "Maggie D" Your theory hinges on the proposition that it is a folk-procesed version of Maggie D.   (If it is not, then it is just a made-up name and does not support the contention that "Maggie D." is the original name.) So we'll go with the assumption that it is, which means that there must have been a song that said "Maggie D." for someone to have misheard.

Now, your own "laws" state that the simpler will trump the more complicated and the familiar will trump the strange. Seems to me "Maggie D." is simpler and more familiar than "Maggadee," so it's strange to begin with that "Maggadee" would come into existence at all. But that it should survive while "Maggie D." doesn't? This goes against the same logical "laws" that you use to derive "Polly Ann" from "Margaret Dabney."