Joe,

What makes you say that "Muddy Waters, who had evidently learned the song from Estes rather than from Maceo?"

Otis Spann played with Maceo before he played with Muddy Waters, so it seems likely that Muddy picked it up from Otis, who picked it up from Maceo (although it also seems likely that Muddy was familiar with the Maceo version which was pretty successful.)

Brennan