Have you found the family in the censuses? If I have my matchsticks lined up correctly in 1871 the family are in Wemyss, Fife but there seem to be more children namely William, Rachel and Robert. William Junior is a tinsmith and Rachel is a hawker. She was born in Banffshire. Robert is aged 2 and said to have been born in Kilsyth ( SP has birth with mmn McMillan which matches).
As far as I can see the family are in Dysart in 1861. Another son, Alexander, is recorded aged 9 b Glasgow. In 1881 and 1891 they seem to be in Lochee, Dundee.
In these censuses William's place of birth does seem to vary but Mary's is consistently showing as Ayrshire but I do feel that they are all showing the correct family.
Have you found a death for William and Mary after 1891?
I cannot see the Isaac you refer to in your original post. ( There is a James showing in the 1881 census). Yes they moved about a lot but do wonder why they would marry in Aberdeen. I would not rule out their being just one couple.
William
The William I believe to be my William's marriage certificate is 21 Feb 1862 in Aberdeen. Parents recorded as George (tinsmith) and Betsy m.s. Townsley. The death certificate that I believe to be for my William is 13 Dec 1903 in Edinburgh. Parents listed as George (tinsmith) and Bridget m.s. Townsley. 67 years old in 1903 puts him at about 1836 birth. The 1862 marriage certificate has him as 29 (although it looks like 27 had originally been written) which puts him at about 1833-1835 birth. If the 1851 marriage listed on some of these children's birth certificates is to be believed, then "my" William would have been about 15 at the time of his marriage (depending on which age you think is more accurate).
The children in question are:
David b. 1858 Dysart, Fife
Mary b. 1860 Dysart, Fife
George b. 1863 Markinch, Fife
John b. 1865 Dysart, Fife
James b. 1867 Cadder, Lanarkshire
Robert b. 1869 Kilsyth, Stirling
Isaac b. 1873 Lochee, Dundee
I didn't question any of them because the parents were William (tinsmith) and Mary McMillan and the years all seemed to line up. That was until I noticed, for example, George's 'when and where parents married' is "February 1851 Aberdeen". John is "1862 February 21 Aberdeen". Robert (in 1869) is "about 19 years ago Aberdeen". Isaac is "1862 February 21st Aberdeen"...etc.
So I said ok there are clearly two different people here. Two different sets of William McKenzie and Mary McMillan's. I haven't looked into census records for them yet, but will have to do that in order to sort this out. It's just odd because George and Robert and Mary are family names my ancestors used repeatedly. Which is why I think there must be some sort of connection here. Perhaps the older William is a younger brother of "my" William's father George?
They were travelling tinsmiths which makes identifying people based on location next to impossible because they were always on the move. I suppose census records would be best bet at distinguishing the two groups.
Also intersting you see a William, son. I have a William in my tree, but he is "blank" as I haven't been able to find any information on him. The only reason I know he existed is because William's death certificate is signed for by a son named William.