"It was strange, though....in the footage and discussion of The Weavers, I don't believe Fred Hellerman was ever mentioned by name. "

I found that odd as well. I chalk that up to a production oversight rather than an intentional slight of Fred Hellerman.   I saw a preview copy of the film last fall and many of the supered titles were missing.

By the way, I have heard through the grapevine that this documentary might be released as part of a package with Jim Brown's other documentaries including "Wasn't That a Time" and "Isn't This a Time". I do not have that confirmed, but I heard it from a usually reliable source.