Hi again,
If you like the looks of the family in my last post, go back to 1841 and indeed, as you suspected, Sarah was the second wife.
George Newal married Sally Dowie 3 Mar 1846 in Crossmichael, Kirkcudbright
The mother of the Newal children who interest us was Isabella:
1841
Lochsmadde, Crossmichael, Kirkcudbrightshire
George Newal, 70, born Ireland, Farmer
Isabella Newal, 65, born Kirkcudbrightshire
George Newal, 20, born Kirkcudbrightshire
Jean Newal, 2, born Kirkcudbrightshire
Delving a little further we see that she was Isabella Lindsay. Through the intervention of some benevolent spirit, daughter Mary's christening appears on the IGI:
Mary Newal was christened 16 Nov 1812 in Balmaghie, Kirkcudbright
Father: George Newal
Mother: Isabella Lindsay
This appears to be one of those parts of Scotland where the online IGI only has female births. In fact, Mary had at least 3 brothers, one also born in Balmaghie (George in 1814) and two born in Buittle (Robert in 1805 and James in 1806). There may be more, given the gap in the middle and given a tantalizing Charles Newell, born in Balmaghie who keeps popping up in the censuses, but those are all whose births I found.
There is no convincing Mary Newal in 1841, but several possible Mary Thompsons, so I think she was probably married before then. Unfortunately, Ancestry didn't index the marital status, and FreeCen hasn't finished this county yet, so there's no easy way to check which of the several plausible Mary Thomsons are married. None are living with a Joseph. But there are two in Balmaghie, one working as one of 6 Ag Labs for a farmer named David Bean, and one working as a servant in this stately home:
http://www.visitsouthernscotland.co.uk/ ... i2681.html
The Johnstone family was living there in 1841, with 7 children the last two twins, so they needed a bit of help.
If either of the two Balmaghie Marys are yours, she would not have been far from her parents in Crossmichael. She might even have left the children with them while she earned the money to support them. It's not clear which of the Joseph Thomsons might be George's father...
There is a
Joseph Thomson, parents Joseph and Mary Newal, born in Crossmichael in 1844. He is the only child born to parents with those names between 1830 and 1854 in all of Scotland. So... I am leaning towards the idea that George started out life as Joseph, and for some reason got changed to George. We hear no more from Joseph and Mary and it is possible they both died soon after George's birth. Or maybe Mary died at his birth and Joseph did a runner... and the grandparents decided they would rather call him after Grandpa.
Or... since the one born in 1844 would have been neither 9 in 1851 nor 20 in 1867, and since there is no sign of a John being born either and there he is too in 1851 with his grandparents, it may be that Joseph was the only one who made it into the register, and perhaps he and the mother died shortly after his birth. All speculation, of course

I would be tempted to get the film of births in Crossmichael and scroll through it page by page...
All the best,
Sarah