Looks to me as though two or three of her have gotten all jumbled up together, along with various "husbands" and "children".
The "Margaret Campbell" whom Thomas Boyd, 6th Lord Boyd, married apparently died young, possibly before 1568, because he had at least one more wife (Elizabeth Wallace, who survived him).
There was a Margaret Campbell who did NOT marry Thomas Boyd, but DID marry 1) Robert Montgomerie of Eglinton and 2) Ludovic Stuart, 2nd Duke of Lennox. (She is also known as "Jean".) This one was apparently born later (c. 1560?) and died before 15 December 1610. http://www.thepeerage.com/p528.htm#i5276
Same source claims that it was *Marion* Campbell (sister of Margaret) who married Thomas Boyd.
