Thanks for your comments about the book, Alec. The barons of the Mearns (or leading lairds of Kincardineshire), one of which was Alexander Falconer, were able to get away with the ritual murder of the sheriff in 1421 (by killing him and boiling his body, each of them drinking a spoonful of the broth) and then obtaining a pardon from the chief of Clan Macduff by each giving nine cows and a heifer at the Cross of Macduff. Apparently they invoked a very archaic Scottish law. This was recorded in the 17th-century document in the Barclay family papers. Since the landowners must have been united in opposition to the sheriff, there was probably nothing the King could do to prosecute them, especially if they used the archaic law as a cover for their deed. But it's interesting to know that even that late they were familiar with pre-Christian laws.
Of course there isn't enough in the way of documentation to show how they were related to Macduff. It isn't until really the late 15th century until Scottish records become sufficient in quantity to trace families. It seems probable to me that all or most of the Falconers in Kincardine and Moray are descendants of Ranulf le Falconer (fl. c.1210), but the lines which broke off before about 1500 are impossible to trace. I don't know if servants or retainers took their names from their masters, but I think that the number of Falconers is low enough to suggest that they were in fact genealogically all connected.