Sorry this response has been a long time coming - I've been diverted by other things and have only recently got back to this.
The marriage of one Roderick Gollan to Mary Chisholm that you mention is very interesting.
My ancestor, Roderick Gollan b~1820, died Victoria, Australia (according to his death certificate) in 1898, 78yo, parents George Gollan, bootmaker and Mary Gollan formerly Chisholm. The informant on the certificate was Roderick's son-in-law.
There's a few possibilities as you say - given that Mary Chisholm appears as Roderick Gollan's wife, it may well be that the death certificate is misinformed.
First possibility is that my Roderick is the son of Mary Chisholm and George Gollan. However, you haven't listed a Roderick as one of Mary Chisholm's children.
Next, there are two George children - the first one is a shoemaker which is the same profession as listed for my Roderick's father George on the death certificate. You state that he married Isabella Innes (died Conon Bridge). Do you mean George died or Isabella died? In either case, perhaps my Roderick is the son of George shoemaker and his mother was incorrectly identified as his grandmother Mary Chisholm (who perhaps had a hand in raising him if his mother died).
If this is the case, the dates would suggest that Roderick who married Mary Chisholm would be the brother of Thomas who married Janet McKay. George shoemaker would therefore be cousin to both of Thomas' children Ann and William.
There is also the second last child, George Ross (Ross being the middle name???). It is also possible that he took up the shoe-making trade and is my Roderick's father, although this is the less likely option as far as I'm concerned.
Many thanks for posting Jackie - this is a wonderful lead for me, and a welcome breakthrough for my Gollan efforts which had stalled.