"a Gaelic speaking Highlander using Lowland dialect or accent"

I don't really see what would be so unusual in that. In versions of the ballad it is one of the Highland clan elites visiting Edinburgh. We're not talking ordinary MacDonald clansmen so surely some of the Highland chiefs would have been bilingual? Speaking not only Gaelic but getting by in Scots too. How else would they have communicated with ordinary women in Edinburgh?