There is no tune in Gore's Scottish Fiddle Music Index called "Mistress"-anything. That wasn't used as a woman's title in 18th century Scotland. It was always "Mrs MacLeod of ...", though there were several spellings of "Raasay".

It didn't turn into a G tune "when it left Scotland" - it stayed in A, and the same way round, in England and Cape Breton. The backwards G variant is Irish.