I have just finished doing some preliminary work on the Oliphants in Roxburgh. I have merely gathered as amny threads as I can, you are welcome to a copy of the spreadsheet so far if you wish.
The other point is that some of the Oliphants from Roxburghshire moved to Missouri and with them the first name of Ralph was popular. There have been quite a number of Ralph Oliphants in the USA since, many of whom I assume are related.
Curiously, there was an Andrew Oliphant who married an Agnes Fiddes in Maxton in 1728. Then there was a Jenet Oliphant who married there in 1759. I must have missed the Mary born in Sough Dean.
There were a number of identifiable Mary Oliphants at that time, including one in Haddington. Do you know anything about the John Douglas whom she married?