I have looked through the Scottish Peerage and cannot find any Lady Marion Stewart born in 1460 married to any Robert Cochran. There is an Elizabeth Marion Stewart , Countess of Lennox b. 1476, but she was never married to any Robert Cochran. Likewise there is a Marion Stewart b.1530, daughter of Elizabeth Semple and a John Cochran, who married a John Cochran, but not a Robert Cochran.
Where did you get your information from ???