Could I check Mary's birthplace, please? You say
Mary b 13 Sep 1826 Forres Morayshire
but all the census entries have her born Knockbain circa 1830
I'm wondering if this is Malcolm in 1881 with a relative, Alexander Cameron:
1881
55 Thomson Street, Govan - multi-occAlexander Cameron, hd, mar, 42, house carpenter, b. Urquhart, Rossshire
Elizabeth, w, 33, b. Glasgow
Alexander, s, 9, b. Partick
John McM, 4, b. -do-
Andrew D, s, 1, b. -do-
--
Malcolm McKenzie, lodger, 21, spirit dealers assistant, b. Urquhart,
Charles Frazer, lodger, unm, 27, -do-, b. Morayshire
Added - this looks a better possible for Mary Cameron's birth/bpt :
https://familysearch.org/pal:/MM9.1.1/VQ73-YYY