I agree with you all around. I didn't mean to imply that there was a well-developed concept of the "bluesman" in Robert Johnson's heyday, and that Leadbelly and Mississippi John didn't fit it. Merely that they sit uneasily within our modern concept of "bluesman."
I also agree with your question about Big Bill's fiddle, and extend that to Leadbelly's accordion, which was his first instrument. How cool it would have been if he'd played it more often in his professional career! As it is, you can hear it on a few tracks of the Folkways collection.
Hootenanny, I don't know if Big Bill ever adopted overalls for stage performances, but Leadbelly definitely did. He also preferred to dress sharply in double-breasted pinstripe suits, but for folk festivals and other appearances he sometimes affected overalls and sat on hay bales; I've seen the footage. In the same way, the Clancy Brothers wore their Aran sweaters and sat around a fake kitchen table placed on the stage. It was part of the festival scene then.