Have you considered that maybe John and Sarah weren't married?
I wonder if this is Sarah in 1851 at Dunoon with a couple of years shaved off her age -
William Mollison 53
Rachel Mollison 55
Elizabeth Mollison 24
Isabella Mollison 21
Alexander Mollison 74
Sarah Wilson 29 born Moffat, Dumfries
Archd Porteaus 15
James Gallouay 29
David Watson 38
James Buchman 25
Hugh Munay 23
Mr Mollison is the Inspector of the Poor and apart from the Mollison family all are described as visitors.
Maybe the Kirk Sessions would reveal something about John's birth.