The names Musgrave and Barnard and their derivatives occur in place names all over northern England and southern Scotland which isn't surprising as through intermarriage the nobility acquired castles, towns etc all over the place. The Percys of Northumberland for instance had all sorts of land in various parts of England and probably Scotland too after the Union. Ballads of this sort had their heyday in the 16th century but as has been stated the stories they tell could date back to much earlier times.
As for conjecturing dates of origin from internal evidence, yes this is great fun, and marvellous if you can turn up something new and concrete. Try working on 'The Cruel Mother' and 'John of Hazelgreen'