Regarding who built the mountain dulcimer Kathy was playing, I have received information from my young music friend Matt Searles about the mountain dulcimer-- here's the reply Matt received in response to an email he sent to Carol:
“The dulcimer Kathy plays in the documentary is one I made for her. I had never made a musical instrument before that project, and sadly, never made one after that either, since I moved away from access to the woodworking tools I used to build the dulcimer shortly after that. I did get lots of help from a real luthier for the project, so it turned out well.”
Matt is who introduced me to the music of Kathy and Carol and I am grateful to him!