According to Burke's Genealogical and Heraldic History of Peerage, Baronetage and Knightage
1914 Pg 706
https://www.google.com/books/edition/_/RVggAAAAYAAJ?hl=en&gbpv=...
the father of Alexander governor of Berwick was Alexander, whose father was Serlo, son of Bertrand.
If Burkes is correct, Christopher Seton and Agnes Dunbar are not the parents of Alexander, the father of Alexander governor of Berwick.