I entirely agree with your perspective on the "n-word", and other words that have come to have a pejorative meaning, whether or not that was their original intent. However, I don't think I agree with where you are going with dialect in songs. Where the dialect originated with black-faced performers, whose use of that form of the language was only to ridicule {insert adjective of your choice here} people, then I'm with you entirely. A statement such as "this dialect originated in music halls and has no real connection to real people" is completely appropriate.
On the other hand, there are songs that are commonly sung in Irish, Scots, Swedish or German accented English, or that American dialect commonly called Southern. How is this different from singing a song in the form that it originated, among enslaved Africans or their immediate descendants, enslaved or not? I can't think that your intent would be, to give an extreme example, to translate all of Huddie Ledbetter's songs into contemporary English.