Ross's hunch, back in 2003, from the 1841/2 Grenville census, that David, son of David and Jane (WILLIAMSON) FAIRBAIRN was married earlier than to Charity, looks accurate. At that time the census merely showed how many people of what sex, age and marital status were in the household, and it implied that David and a young wife were in the house, although David and Charity didn't marry until 1845, and her age didn't match that of the other married female in the house besides Jane, David's mother.
While looking for something else, as so often happens, I stumbled upon a marriage in 1841 of a David FAIRBAIRN and Prudential ARNOLD in the Presbyterian Church records in the Drouin colleciton showing they were married at "Grenville, Ottawa" by William MAIR, Minister of Grenville & Chatham, witnesses were John BLAIR and John TOPLAND.
In the same set of records were David and Charity's marriage, and the baptism of their son David.
The FAIRBAIRN DNA project is looking set to examine more closely the "cousin" relationships that have so taxed my brain and genealogical inventiveness. William has joined as a rep. of the line of John A FAIRBAIRN Snr of Stillwater, whose son John A of Hoquiam is the one these relationships were all reported to: Angus s/o the above David and Charity (WALKER) F.; Albert who emigrated from Sunderland to Hoquiam, an unlikely choice about 100 years after John Snr's parents had left Scotland; and Walter to Clarissa.
How I currently see these relationships can probably best be viewed starting at the dna oriented descendant chart for David and Jane (WILLIAMSON) FAIRBAIRN, and from the charts listed on William's dna page, not that his results are in yet, as the kit has only just been ordered, patience (not one of my strong points)!
It is obviously a dna day today. We now have a DAVIDSON representative who has been enrolled in the DAVIDSON project (not one of mine), that just might help us break thru the brick wall of finding more about John DAVIDSON. Although I strongly suspect he was the illegitimate son of the mother Ann COLLINS shown on his death cert, his father was more as like a DAVIDSON given the naming conventions of the time.