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!  

Oh, thanks for the information, @leny-sue!  

There is one other more recent one titled "Keepsake". Yes, their harmony is perfect.

@leny-sue I'm glad to know that and imagine others are, too!  I'm not sure but the circa 1965 album may have been their only LP.  I love their sound!  

The most recent videos I found were from 6 and 7 years ago.  Here's one from 7 years ago:

That was great! I remember them from the Ash Grove and other venues in Southern California "back in the day." I especially remembr their versions of Carter Family songs. and an autoharp with changeable bars. I wonder if they have stayed active since that video was made.  

Thank you for sharing this, Robin! I don't recall hearing Kathy and Carol before...but I wish I had!

At the 5:30 mark in the YouTube video, you will see Kathy Larisch play and sing in a style very reminiscent of the style of Jean Ritchie.  Are any of you familiar with the duo of Kathy and Carol?  I'd never before heard of them until a young music friend shared a bit of their music with me today.  I'm curious as to who may have built Kathy's mountain dulcimer, too.