The missing chord that Mary played was Dsus4. She played a lot in open tunings, and used a lot of suspended chords in those tunings. (She had a page about this on her website, which you can locate in the Internet Archive's Wayback Machine; I link to it from https://www.pelicancrossing.net/tunings.htm.)

"...hold me down on the road" I believe is "somewhere down the road", which makes a lot more sense! At any rate, that's how I'm singing it.

wg