How did you find out that Roderick was born in Dingwall, and where does his date of birth come from?
Looking at that 1851 census, where Roderick McDonald, aged under one year, is with his grandparents in Dingwall, it occurs to me that he was possibly illegitimate.
That might then be the explanation why there is no record of the baptism of a Roderick born in Dingwall on 1 December 1850.
So which of the Dunbar daughters was Roderick's mother?
Colin Dunbar and Ann Urquhart had nine recorded children: Simon Mackenzie, 1816; William, 1818; Betty, 1821; Ann, 1823; Robina, 1828; Johanna, 1830; Colin, 1833; Proby, 1837; Elisabeth, 1837. There is a gap between Ann and Robina long enough for at least one more child.
In 1841 the family are in Maryburgh, including Colin and Ann plus Ann, 17; Jacobina, 13; Johanna, 10; Colin, 8 and Eliza, 4. There's no possible candidate in the census for Betty, so presumably she had either died or emigrated and can be ruled out.
In 1851 there are Colin, 60; wife Ann, 60; daughters Ann, 26; Hannah, 18; Eliza, 13; son Colin, 16 (all unmarried) and grandchildren Betsy McMillan, 5 and Roderick McDonald.
by 1861 Ann Sr is a widow and with her are her daughter Ann McMillan, widow, 36, and grandchild Roderick McDonald, 10.
I presume that Hannah is Johanna, but is Robina Jacobina?
Eliza can be eliminated as she is too young.
Ann married Edward McMillan in 1845, so is presumably the mother of Betsy, but why is she listed as unmarried in 1851? If she was widowed before 1850, she could be Roderick's mother.
I have found no trace of Robina/Jacobina either after 1841.
Jo(hanna)h was just old enough to have been Roderick's mother.
So I speculate that the most likely candidate for Roderick's mother is Ann Dunbar, widow of Edward McMillan, and the next most likely is Jo(hanna)h, with Betty and Robina/Jacobina as unlikely.
On the other hand, I see from the census that he named a daughter Robina.
But as Roderick married in England, his marriage certificate won't give the name of his mother, and if he also died in England his death certificate will be even less informative.