Yes, I think you need to go back to basics altogether.
You have no solid evidence to tell you where or when either Peter or Isabel was born, or who their parents were, or when or where they died. I don't know what you mean by
Source Citation for Scotland, Select Births and Baptisms, 1564-1950 but whatever it is, it is not an original record and cannot be relied on.
Scotland's People does have a record of a baptism of Isabel, daughter of William Shand and Margaret Young, in the parish of New Spynie on 29 January 1787, but there is no guarantee that this is your Isabel. She could just as easily be one of the 10 other Isabel/Isobel Shands whose baptisms between 1775 and 1790 are in the records at
www.scotlandspeople.gov.uk, or another one altogether whose baptism record has not survived.
If, and it's a big if, Peter and Isabel followed the naming tradition, you would expect Peter's parents to be William and Isabella, and Isabel's to be Peter and Margaret. But if both their mothers' names were Margaret and/or both their fathers' names were William, the naming tradition doesn't help.
Also, is there any actual evidence to suggest that Peter and Isabel ever lived in Aberdeen? From the absence of any likely-looking listing in the census it looks to me more as if they could both have died in Rathven before the 1841 census.
There is a possible candidate for the labourer who died in Aberdeen in 1852. This is a Peter Copland, aged 68, a day labourer born and living in Monymusk. It is possible that he was taken to the Infirmary in Aberdeen and died there. If you want to follow him up, the Infirmary records are available to consult in the Aberdeen University Library and they tend to be rather more informative than the church burial records.