Hi Christine,
I've been researching my wife Sheila Mackay's family and you're definately related.
George Levison Gower Mackay born 3 July 1835 Dornoch, is the George no 6 in your list of children. He married Elizabeth Hardie 6th July 1858 in Old Meldrum. One of their children was Robert Mackay, my wife's grandfather.
John Mackay (no 8 in your list of children) and Mary Murray married 27th January 1877 (or 16th February 1877, depending on which record you believe) in Rogart, as you know. One of their children was Margaret Anne Mackay, my wife's grandmother.
So infact Robert and Margaret, who married 3rd September 1910 in Inverness, were first cousins and my wife is related to 2 of the children in your list!
Do you have any more details that you can share on Donald Mackay and Margaret Nicholson and their ancestors? Or on any of the other children in the list and their families? I've been trying find out about the Mary Murray who married John, but whilst I know her parents were Angus Murray and Janey McIntosh, I can't find her birth or their marriage records.
Regards,
Mike