I'd definitely vote for the Hawaii origin theory. Yodeling turned up in country music about the same time as the steel guitar, which was invented in Honolulu. Traditional Hawaiian music involves a lot of falsetto and yodeling, and although I tend to doubt that there were that many Hawaiian entertainers running around the Mainland during that period, there was the radio show Hawaii Calls, and probably a whole bunch of country boys returning from Navy service at Pearl Harbor and Army service at Schofield Barracks. I suspect that was who brought the steel guitar back to the mainland, along with yodeling and falsetto singing, as in Cattle Call.