I have TODAY found information on the mysterious DUNCAN OLIPHANT that seems to close a few doors.
1. From the Oliphant DNA results, it appears that Duncan comes from the branch of the family centered in Culteuchar, Scotland, not the Gask line who later sided with the Jacobins and were exiled. Information confirms that "The chieftan of the House of Gask, Laurence Oliphant and his son fled to Sweden and finally took refuge in France," who sometime after the uprising 1745 to 1746. William Oliphant of Culteuchar was the 10th Lord Oliphant. Thus, he could not be of the same family that fled to America in the 1600's, and they are not descended from this peerage.
2. As to David Oliphant, another possible father, there is a David who was granted in 1675 rights to his father's estate by royal decree, the father being William Oliphant of Culteuchar. Again, this points to a Lordship.
3. Duncan and this David possibly share a common "David" ancestor. As has been pointed out, Duncan is sometimes a nickname, and at least two cousins are found in America about the same time. Family Search pedigree files have about 30 David Oliphants, but these are actually the two same individuals. One is said to be son of Lord Laurence, the other shown to be married to Janet Gilchrist. Although the birthdates of these two appear to be the same, the parents mentioned for Janet's spouse are either missing or not the same Lord Laurence, who is a descendant of King Robert I known as "the Bruce." It does not seem likely these two men are the same person because some of those pedigrees mention a David as Duncan's father.
4. His wife is probably not Mary Garrison, but Mary Guyon from a Huegonot family.Duncan is not the son of Lord Laurence. He may be the son of a David Oliphant, but it will not be the David Oliphant of Culteuchar, Scotland, son of William, again a probable member of the peerage who lived decades later than Duncan.
Duncan seems to have been born in Scotland and died in America. We still don't know who his parents are, but it seems we can eliminate these two most mentioned candidates. I'd love to get feedback on this.