Montana Slim Carter and Goebel Reeves also used the yodel in their recordings and I would think both would fit some kind of definiton of "folk". I kind of enjoy Montana Slim, but find Reeves yodels too quirky and annoying. Arkansas natives Patsy Montana and Elton Britt were well known yodelers in their day and had million selling records. Britt's million seller did not require any yodeling, however.

Has the question ever been answered as to how the yodel was transmitted from Europe to the American south and west? Or did the yodel develop independly in this hemisphere?

Yodeling did find some new life in modern country music when Leanne Rimes learned a little bit from Janet McBride and put it in her version of "Cowboy's Sweetheart". That yodeling was a bit rudimentary, but Janet, herself, is excellent at the practice of yodeling.