Sonja,
Firstly, I mis-read Samuel’s father’s name on the 1848 marriage certificate. You are quite right it’s Solomon. It needs looking at closely but that’s what it is. And that evidently ties in with the family gravestone.
John Campbell was 71 when he died so born around 1810. If Samuel Campbell died 21.3.1893 aged 80 that makes John & Samuel of an age to be brothers. But I don’t know of any records that might confirm that with certainty. That they shared the farm for many years does suggest they were closely related.
John’s son Gabriel married in 1883:
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1883/10939/5997630.pdf
There are some trees on Ancestry (eg the Mayrs tree) that have William John Campbell & Mary Mairs (spelled Mayrs). They show William John’s father Thomas as being married to Margaret Simon with Thomas dying 20.2.1889 and Margaret dying on 26.3.1881. Both those deaths were in nearby Balteagh townland. Thomas married Mary Simon in 1851 when he was also living in Balteagh. That certificate names Thomas’s father as George. So if this is the Thomas who also farmed in Kiltinney then possibly he isn’t descended from either Samuel or John at all?
Separately, the Cunningham-Zdrazil tree is one of 4 with Solomon Campbell and Margaret . It lists them having a daughter Martha (1796-1831). And as you have mentioned, Samuel and Nancy had 3 daughters Elizabeth Campbell 1850 – 1928; Letitia 1854 – 1926 & Margaret Jane 1858 - 1937. No sons are listed.
Elizabeth married William Moore in 1869:
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1869/11435/8200650.pdf
Family in 1901:
http://www.census.nationalarchives.ie/pages/1901/Londonderry/Somerset/C…
Margaret Jane married William Douglas in 1880:
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1880/11024/8031084.pdf
Family in 1901:
http://www.census.nationalarchives.ie/pages/1901/Londonderry/Coleraine/…
I can’t find a death or a marriage for a Thomas who is in the Valuation Revision records for Lower Kiltinney from 1883 – 1898. So possibly he is the Balteagh Thomas. If the Mayrs tree is accurate and Samuel & Nancy had no sons, then they may not have had anyone to take over the farm. So possibly Thomas was a relative who was willing to take it on, and it then passed to his son William John.
Gabriel, who inherited the Kiltinney farm in his father’s will is in Balteagh Lower by the 1901 census, so something must have happened that made him move. Some deal has been done which led to the farms being swapped around. Gabriel & family in Balteagh Lower in 1901:
http://www.census.nationalarchives.ie/pages/1901/Londonderry/Letterloan/Balteagh_Lower/1517597/
Expanding a little on the Kiltinney farm in the Valuation revision books, I mentioned that 3a went from Samuel to the mysterious Thomas in 1883. After that it went to William John in 1893, 3b went from John to Gabriel in 1883 and then to Hugh Gage in 1891. In 1898 it’s consolidated and taken by James Cooper Mayrs but sublet to William John Campbell (who you will recall married Mary Wallace Mairs/Mayrs).
You might want to have a look at the Valuation Revision books yourself:
https://apps.proni.gov.uk/Val12B/Search.aspx
So there’s apparently been some sort of deal whereby Gabriel acquired a farm in Balteagh Lower and William John got the one in Kiltinney Lower. But it doesn’t tell us precisely who Thomas was and how he was related to Samuel and John..
A general note about Griffiths by the way. The clerks revisited properties every couple of years to update the records. Some of the dates can often be out by a year or two. For example we know John Campbell died in 1881 but the change to his son Gabriel isn’t recorded till 1883. That’s normal. The dates need to be viewed flexibly. They are often indicative rather than accurate.
There are a lot of Campbell families in the area (4 households in Balteagh Lower in 1901 for example) and it’s easy to get them mixed up. Here’s Solomon Campbell in Kiltinney in the 1831 census. At that time there were 4 males and 4 females in the household, all Presbyterian. So it’s possible he may have had 3 sons and 3 daughters (though some of the people might have been other relatives. We have no way of knowing):
http://www.census.nationalarchives.ie/pages/1831/Londonderry/Coleraine/Mecasquin/Killiney/16/
Next door was a Thomas with 2 males and 1 female, and next door to that was James with 3 males and 3 females:
http://www.census.nationalarchives.ie/pages/1831/Londonderry/Coleraine/Mecasquin/Killiney/17/
http://www.census.nationalarchives.ie/pages/1831/Londonderry/Coleraine/…
Crossgar Presbyterian’s baptism records start in 1839, marriages in 1846. I am not sure if the church opened in 1839 or whether it’s just that they have no earlier records. Macosquin Presbyterian has baptisms from 1823 to 1845, 1860 and then 1867 onwards. Copies of both are in PRONI. You might find the children’s baptisms there which may help.