Having just done a quick scan of the list of Child Ballads I see no mention of "William and Lady Marjorie" If the mores of the time be known a good case can be made out for this ballad being pre C.1450.

Another I see no mention off is "Lord Spynie"--the story behind this one having 2 seductions 1 manslaughter and one assasination all dating to the year 1607.