>The Am chord can be seen as an ellipsis of Am7 alias C/A. In my post above, we can assume the same melody C B A G above a constant chord C with alternating bass notes C and G - absolutely normal in European-American folklore.>

I disagree. Cmaj7/G has C as its root, not A. The descending bass line from a C major chord
was not used in banjo music pre-Pete Seeger. If anything, the banjo in accompanying vocals in traditional American folk music would duplicate the melody line. The descending bass line, C,B,A,G was not part of the traditional American Appalachian or Blues tradition, but a device used in classical, popular and jazz music.

Pete brought in the counter-line bass movement from the guitar and piano into his banjo accompaniments picked up by his sister Peggy.