My mother-in-law was a Cammock. As far as we can discover the family moved from County Down in 1830's and 1840's (We can find some but not all on 1841 census). During my research I came across a DAVID CUMMOCK on the 1871 census. He is not connected in any way that I can discover. He (age 26, County Down) is living with his wife Isabella (27, County Antrim), daughter Mary (7, Warnhill Ayrshire), son Edward (3, Dreghorn Ayr) and son Alexander (0?, Hamilton Lanark). There is also his widowed mother-in-law Mary Quin (55, County Down) and a Samuel Quin (14, County Down) who is noted as son-in-law, but should probably be brother-in-law. David and Samuel are both coal miners.
I also found a marriage between a David Cummock (23) and an Isabella Quin (22) in Dreghorn on 16 May 1867. His parents are given as Alexander Cummock (labourer, deceased) & Jane Gibson, whilst hers are Edward Quin (labourer) and Agnes Minagh - not Mary. I have not found any of these individuals on the 1881 census, nor have I discovered any death registrations: I have looked fairly closely for civil registrations of all births, marriages and deaths in Scotland of Cammock variations as it is not a very common name.
I think there is a good probablity this is the same chap as your David but the name of the putative mother in law is puzzling as is the presence of the daughter Mary born before the marriage. As there is no immediate connection I have not investigated further.