My own research (from over 20 years ago) on the origins of this myth...go back to those wacky Disney animators. I was told many of them started the Walt being frozen story by complete accident, as mainly a reference to how the "after-Walt" management team was acting. "They were acting like Walt was frozen...afraid he was going to pop-up someday and critique what we had done!"
This "frozen" business leaked out and was literally misinterpreted and propagated.
The first instance of anyone picking up and running with the story in print (that I could find) was in 1969 in an issue of Ici Paris magazine (a sort of National Enquirer type tabloid in France). This is also mentioned on this Snopes page:
Here in the U.S. it really blossomed into a full-blown urban legend after a major mention in the National Lampoon humor magazine sometime in the early 1970's.
From there, additional items were added to the "after death" myth, like that Walt had filmed a whole series of presentations before he had died that were shown yearly to the Disney Board of Directors. "At this point...you should be here with EPCOT." That sort of thing.