As a matter of interest Karen. Has any of your McArthur male line been DNA tested please. Either Familytreedna or ancestry.com autosomal and My Heritage autosomal..  If they have you have then answered my question as we have no McArthur matches, but if not, hopefully one day they will test as  to date familytreedna have about 35 McArthur members in their McArthur or Arthur project and we dont match with any of their McaRthur males there and dot have for the autosomal companies to date. Also the location for our McArthurs who have tested to date are from the Highlands, Keay in Caithness and other ares of Caithness, Sunderland and one inLismore. We have done the Y111 marker test and th BigY700.  Have already put the naems that are coming up and like your fellow there has been a name change along the line  at least prior to the mid to late 1700's. It is looking as if McArthur was an alias, inherited or other surname. Cant find anything onthe father Duncan McArthur, though downloaded a website with farms, that listed tenants at Breadalbane and are of interest.  They listed tenants by the names of McArthur for the year 1767, it was a survey.  I note another one  for 1780 is available as well.

regards

Edie McArthur