It almost sounds as if Roderick and Alexander are one and the same.
All very curious...........
Some questions
1842 marriage record. Is there any information about Roderick McKenzie? His residence? His occupation?
1855 birth record. What are ALL the details given about Alexander McKenzie? Age, occupation, place of birth, etc.
Who was the informant for this record?
Do you have any birth records for the other children? What information about the father?
Do you have Alexander McKenzies death certificate? Please supply information.