Hi Southpaw58, just clicked through on a notification!
Ah! John Brash & Mary Wallace.... The couple that every descendant has got stuck on. I found it too difficult too, so this is stirring the embers with a stick hoping something will result! I take it the son you're referring to is John Brash recorded as born 26 Aug 1792 Abercorn (and he married Christine Clark).
Untangling all the John Brashes has been a complete headache for me too - in fact I wonder whether I've succeeded. There are other factors in the building of my Brash edifice, but one has been to distinguish between Brashes in Abercorn who were wrights and who were smiths. That is a very subtle distinction but it's had its uses. This occupational distinction comes into play later - read on.
No, I have nothing on John Brash s/o John Brash (smith in Newton) & Christian Greenlaw. What I have decided is that his sister Elizabeth Brash is my 4xgtgdmother Betty Brash, making a bit of a judgment on available evidence (there are other candidates locally but none in Abercorn, including ** below). My smith/wright distinction is no help because she married Thomas Shatton who was always a farmer - which in fact implies I should perhaps have made a third distinction: smith/wright/farmer.
Re your John Brash h/o Mary Wallace, I've long had five candidate parents with John Brash * & Janet McComie/McKomy being very much the prime candidates. I have noted John Brash & Agnes Smale (who marr Kirkliston 8 Nov 1717) as candidates to be his parents, such that he would have been born 5 Dec1718 Kirkliston. * You say that this John Brash was a soldier. Any other indications of his or his son's occupation?
Re John Brash & Janet Thornton (or Thorntoun). This John Brash was a wright in Duntarvie then in Newton. Mortcloth Rights records show that he was s/o Alexander Brash in Lynnmiln and that proves him to be s/o Alexander Brash h/o Katherine Findlayson. Which is something definite for once. I have very little on the fate of John Brash & Janet Thornton and their six children.
But so far, mainly so confusing and inconclusive. It gets worse - or perhaps better.... A Mary Wallace, I believe your Mary Wallace, died 26 Jan 1797 in Crawstone, Abercorn, followed by Archibald Brash a child (presumably her child?) on 29 May 1797 in Crawston too.Then I have her widower John Brash remarry to Elisabeth/Betty Brown 6 Jan 1798 in Kirkliston (also recorded in Abercorn) - and this couple then turn up in the 1841 census in Milton, Dalmeny, John Brash being a farmer.
Meantime, they had had a daughter Bathia (later Elizabeth) Brash in 1802 at Crawstone (and possibly a second in 1804).
I then find the widow Betty Brash (née Brown or Broun) over at Easter Duntarvie in 1851, age 78 with a [step] gdchild Agnes Aiken (d/o John Aitken & Marion Finlayson where the latter is d/o William Finlayson & Agnes Brash one of the John Brash Mary Wallace family group). She died, as Betsy Brown, age 81 5 Dec 1852 in Duntarvie, probably Easter Duntarvie. I've been told that 'your' John Brash, who died 1 Apr 1850, died at Duntarvie, hence probably also Easter Duntarvie. Easter Duntarvie is where my Chatties had been tenant farmers from 1787 until 1821 when they moved to Totleywells. I don't know whether this is anything more than a coincidence of place.
In 1841 John Brash was a farmer, but in 1895 a gtgdson claims he was a quarryman. But there's that farming distinction again. In fact, I have temporarily allocated your John Brash (farmer in 1841) with three siblings, those three seemingly all children of one 'John Brash farmer in Newton'. And this 'John Brash farmer in Newton' might be h/o Agnes Thomson (**) having married previously. By this time I'm going round in circles.
How much of this accords with your research?
Note that on at least two occasions, 'Bethia' or 'Bathia' has become 'Elizabeth'. I suppose that's not too exceptional.
A further thing to mention in passing is that Bethia/Elizabeth Brown Finlayson, Marion's sister, married George Chapman 26 Feb 1855 Dalmeny. And he is a gdson of Mungo Chapman & Agnes McKinley. And Mungo's sister Agnes Chapman married Thomas Shatton who is s/o Thomas Shatton & Betty Brash. Agnes Chapman & Thomas Shatton (whose surname became Chattie then Chatham or Chattam) are my 3xgtgdparents. The Brown name keeps popping up.