I think you are over-complicating things.
There are two entirely separate couples who just happen to have the same names.
One Robert McKenzie, born in Aberdeen, married in Aberdeen in 1846 a Mary Ann Devine, who was born in Peterhead. They had at least seven children, including Alexandrina. Mary Ann died in Aberdeen in 1863, and her widowed husband later married Agnes Paterson. He was described as cast iron dresser in the 1861 census, iron founder’s metal dresser on his wife's death certificate, and iron moulder in the 1871 census.
The other Robert McKenzie was a travelling tinsmith. Depending on which census you believe, he was born either in Perth or in Cromarty. He married another Mary Ann Devine in Glasgow in 1848. According to the 1851 census she was born in Edinburgh. In 1871 this Robert was in an encampment in the parish of Cadder with his mother Bridget Townsley, brother Isaac, also a travelling tinsmith, two of Bridget's daughters-in-law, and assorted children.
There is an inconsistency in the names of Bridget's daughters-in-law, who are Mary and Jessie. Some of the children in the census match those of Isaac McKenzie and Harriet Tennant, who were married in Glasgow in 1855. But there is nothing to say that either of them is the wife of either Robert or Isaac, so they could be the wives of other sons of Bridget Townsley.
Robert McKenzie, mother's maiden name Townsley, died in Glasgow in 1887. Have you seen that death certificate? What dies it tell you?