Many of the songs on Jacqueline Schwab’s album of Mark Twain-era songs, arranged for Ken Burns’s documentary about the famed writer, are spirituals or other tunes traditionally accompanied by lyrics. But Ms. Schwab, who will play a selection of those songs as well as tunes from her new album, True Blue Waltz, at the Dance Palace Community and Cultural Center next week, offers unaccompanied interpretations on piano that convey their pathos without the aid of…