First of all, the source of the vast majority original Scottish BMD and census documents is
www.scotlandspeople.gov.uk However I've had a rake about, and it's not looking great for answering your questions.
The record of this marriage is not there, so it seems that it has not survived, if it ever existed.
I see that John and Janet had one child born in Scotland, Jessie, in 1859. This is unfortunate, because in 1855 and every year from 1860 onwards a Scottish birth certificate includes the date and place of the parents' marriage.
There's a household in the 1861 census in Byker consisting of Jessie Pringle, 25, born Scotland; James Pringle, son, 5, born Newcastle; John Pringle, son, 3, born Newcastle; Jessie Pringle, 1, born Scotland. Ten years later they are in Guisborough; John (aged 40) is with the family, and Janet is listed as Jennet, aged 35. Both John and Janet are listed as born in Berwickshire.
(Note that Jessie and Janet are totally interchangeable in Scotland. If you're having trouble finding a Janet, look for Jessie, and vice versa.)
If Janet/Jennet/Jessie was 25 in 1861 and 35 in 1871, she was born in 1835/1836, and therefore 17 or 18 on marriage, not 15, and hence the age on the gravestone is wrong. Also, if John was 40 in 1871, then he was born in 1830/1831, not 1827/1828.
The children in the censuses are James, John, Jessie, George, Thomas and Anne. If (and it's a big if) they followed the Scottish naming tradition you would expect John's parents' names to be James and Anne, and Janet's to be John and Jessie, but it's not 100% reliable.
The Robert Pringle, tailor, who was at Broomhouse Paper Mill, parish of Edrom, in 1841, seems to be living wirh his sister Elizabeth and her husband John Ainslie in 1851. Also in the household is his mother Jane, aged 84. It looks as if he may be the one baptised in Edrom in 1795, son of George Pringle, mother not named. In 1861 he is a boarder in Chirnside, and in 1871 he is boarding with some of the Ainslie family. I can't find him after that, unless he is one of the two 83-year-old Robert Pringles who died elsewhere in Scotland, one in 1874, the other in 1877.
I'll go away and scratch my head and come back if I have any more ideas.