As I cannot find another Helen Dun matching the info I have in the whole of Scotland I will assume that the Helen mentioned on the grave stone is she and follow her as much as possible.
Do
not make this assumption.
It is
never safe to assume that the only person in the available records is the right one. There are many people whose baptism records, if they ever existed, have not survived. But this Helen born in 1805 is not the only candidate.
If Helen had a child in 1838, it would be reasonable to suppose that she was born no earlier than 1790. She is unlikely to have married until she was at least in her late teens. It would have been legally possible for her to marry at age 12, but so unlikely as not to be really worth considering.
There are 25 Helen/Hellen Dun/Dunn baptisms between 1790 and 1807. Even discounting any born after 1801, there are still 16 of them, and there could be as many again whose baptisms are not on record.
I see that the children of John Edmond and Helen Dun(n) are Margaret, 1820; Agnes, 1821; William, 1823; Hellen 1824; John, 1826; James, 1830; Elizabeth, 1832; Robert, 1835; Christian, 1838.
If John and Christian followed the naming tradition, her parents' given names would have been John and Margaret. This is why you need to look at her husband's death certificate - if his parents were William and Agnes, it would suggest that they did follow the tradition.
You also need to look at the original baptisms of all 9 children. If the names of witnesses to the baptisms are recorded, these can be valuable clues to the wider family.