While I loved the movie I have been confused as to why the Coens even let Dave Van Ronk's name slip out. Disclaimer: I learned to sing by listening to Van Ronk records in the late 60s. I was lucky to have heard him perform 5 different times in 4 different decades, the last time my friend was his opening act and I got to meet and share a bottle of brandy with him. He was a god.
My wife, though, came up with a possible explanation. Van Ronk was a consummate story teller. His stories were as good as his music. Now imagine that you are sitting at a bar and he tells you a story and it is so good that you must tell it to someone else. And that person tells it to another and that person to yet another. The movie was based on the fourth guy's retelling, which we all know would be very unlike the original.
The movie also would have been more plausible to those who knew the era if the time frame was 1957 before Dave had achieved some recognition and before the album. Because the character Lywnn Davis wasn't nearly as ground breaking a folk/blues singer as the real thing.
Read the book, it's really funny.