I think you're wrong about Mandela. The song was written in 1960, two years before Mandela was imprisoned. The "black boy ...... " was a symbol of what could happen. Later he wrote specifically about Mandela.
I discussed this with Hamish.
The trouble with the song as a National anthem is that sometimes, say Olympic ceremonies, only the tune is played and as the tune was written by Pipe Major John McLellan, it would have to be called "The Bloody Field of Flanders".
The lyrics are superb but if we are to have a Scottish National Anthem, I would suggest "A man's a Man" as it can be sung slowly with feeling and also speeded up, effectively, as a March.