I've stuck with the convention of using Buchanan as the (married) Surname / Last Name for the wives of these Lairds, because most people seemed to be doing that in this area.
But does anyone have confirmation that changing their Birth Name would actually have been the convention used at the time?
An answer from the inestimable Mr Justin Swanström: "We should not use married names for women in this area of the tree unless we have clear and convincing evidence from contemporary records.
Scottish women normally retained their birth names (including their father's territorial designation) until the 18th and 19th centuries, when the English practice of using a married name became more common. The practice of using married names starts with the nobility and works its way down through the gentry to the common people.
Most of you will have done research in Scottish records, so you already know that it is usual to see wills, deeds and tombstones that give a woman's name as, for example, "Agnes MacDonald, wife of Duncan Campbell".