I take your point about the females possibly marrying.
The one relating to John is not my John as I already have his death cert from SP from 1878, death in Montrose.
Duncan McKenzie married Elizabeth Yule Creden in Fraserburgh 1856 and I have tracked him in 1861 Banff; 1871 Aberdeenshire; 1881 & 1891 Old Deer, Aberdeenshire to his death in Jan 1899 Old Deer. What I have found extremely interesting here with this particular couple is that they adopted Catherine Wishart Duthie (born 1874 Montrose) and she appears on the 1891 census as adopted daughter. Catherine also was the informant on Duncan McKenzie's death cert on SP in 1899. Catherine Wishart Duthie was the daughter of Robert Duthie & Mary McKenzie, Mary McKenzie being the daughter of John Peter McKenzie & Catherine Wishart (already discussed in previous postings). So, Duncan McKenzie was in fact (accordingly to my FTM11 the great grant uncle of Catherine Wishart Duthie).
I must admit that this is a difficult family to trace but I wont be beaten yet.
Thanks in anticipation ...