Going back to basics.
What we have so far:
I have a death certificate of John Robertson ( married Isabella Noble 1851 Daviot & Dunlichity) John born 1831 to William Robertson & Margaret McKay. Death certificate only states Inverness shire for birth place.
William Robertson and Margaret McKay had five recorded children
John, baptised 7 April 1828, Parish of Moy and Dalarossie, Inverness-shire
Donald, baptised 14 July 1829, Parish of Alvie, Inverness-shire
Margaret, baptised 19 December 1830, Parish of Kingussie and Insh, Inverness-shire
Angus, baptised 4 September 1833, Parish of Kingussie and Insh, Inverness-shire
Mary, baptised 17 April 1835, Parish of Alvie, Inverness-shire
In the 1841 census, at Balblair, parish of Moy and Dalarossie, are
William Robertson 45 Farmer
William Robertson 14
John Robertson 12
Margaret Robertson 8
Angus Robertson 6
Mary Robertson 4
Margaret Fraser 25 Female servant
Puzzlingly, William Sr and the four eldest children are recorded as not born in Inverness-shire. However, the names and order of birth of the children look right, even if their ages are not quite right; Margaret would have been 10, Angus 7 and Mary 5 on the day of the 1841 census.
Margaret Fraser must just be a coincidence after all. The Margaret Fraser who was married to another William Robertson is listed with her husband and five children in the parish of Boleskine. She was far too busy with her own family to have taken a job as a servant in a different parish.
So we are left with William, who (because adults' ages were supposed to be rounded down to the nearest 5 years in 1841) was born between 8 June 1791 and 7 June 1796.
In 1851, in Balblair, Moy and Dalarossie, are
William Robertson, 55, born Alvie
Margaret Robertson, wife, 42, born Inverness
Mary Robertson, daughter, 15, born Alvie
Duncan Robertson, son, 8, born Moy
George Robertson, son, 6, born Moy
Catherine Robertson, daughter, 4, born Moy
James Robertson, son, 2, born Moy
Donald Robertson, son, under 1 year, born Moy
Noting the gap between Mary and Duncan, I
speculate that since 1841 Margaret McKay's widower has married his housekeeper Margaret Fraser, and started a new family. His son James, who registered his father's address, may not have known that his big sister Mary (even if he remembered her at all) was only his half-sister. If so, then this is indeed your William Robertson after all, and two different Margaret Frasers married two different William Robertsons in Inverness-shire, one in 1830 (the Boleskine ones) and the other presumably in Moy and Dalarossie.
Other children : Joseph 1838, Margaret 1844, and William 1846 - all born Leith?
These three are the children of a different William Robertson and his wife Margaret Surrene.