I am also researching my MacKay family of Roster, LatheronParish. My ggrandfather was Robert MacKay who married Agnes Lunn of Camster and emigrated to Canada in 1882. Robert was the son of Robert MacKay and Janet McBeath. This Robert was the son of John MacKay and Janet Murray who were also cleared from Kildonan. And, finally, I believe John MacKay was a brother of your William who married Ann Matheson. My family also lived mostly in and around Montreal until fairly recently. I have much data on the Canadian descendants of William and Ann MacKay (obituaries, M.I.s and some photographs) and would be happy to share.