Jay Unger and Molly Mason are included in the Mark Twain credits trailer, so I presume that they will be on the cd as well as Horton and Schwab. In the fine Civil War album, Ungar, Mason and Barenberg handled guitar, Schwab the piano and Ungar did the fiddling and mandolin. Horton played on one track, Dixie. Others who have recorded with Ungar are also credited. I haven't picked up the Twain cd yet but I expect to see them all there.
Ken Burns was born in Brooklyn, 1953. His first documentary, appropriately enough, was Brooklyn Bridge (1981). See www.legacyrecordings.com for more details and a video with him discussing his documentary on Jazz.
Hal Holbrook's stage performance on Mark Twain (monologue) is classic (also filmed for PBS). When you call him an intelligent actor, you are not voicing an oxymoron.