The last voice workshop I went to was great. It was taught by a woman who teaches voice etc. professionally, & is a very well known folk singer. She did shortish workshops, & started by warming everyone up physically, which she said that everyone should do before they perform. This made everyone relax, and put us at our ease, although many of us knew each other, there can still be a stifness about performing in front of each other. We then went on to do a voice warm up with the physical exercises, which stretched our lungs as well as vocals. She came along with very simple part harmony songs, which she taught us, & then went on to give a short explanation of how she writes harmonies. This was enough for us to go away with after a session & I agree with NicoleC that you are hoping to pack a lot into a workshop. Also make it fun & not too much theory, a lot of folk singers sing by ear anyway & don't want to be too tied down by structure theory. Have handouts so that people go away with something concrete from the day eg the songs you have sung, research sites(Mudcat) etc.
Good Luck!