GR2
I have been away busy doing more research but not sure it has totally solved things. The 1918 Wm Davidson death cert above is definitely my relative, as it states him being the widower of Eliza Chapman which I know is correct. The cert is also witnessed by William Wilson, son-in-law which I also know is correct. Wm is listed as 71 at death, which puts his birth at 1847 or so. His parents are listed as Wm Davidson, Ag Labourer, Deceased, and Jane Davidson, ms Barclay, deceased.
Eliza's death cert from 1885 shows she was living at Foulzie Cottage, King Edward when she died. The 1891 census has Wm Davidson a Farm Servant living at Foulzie Cottage. It shows him as 44 and having been born at King Edward. This obviously links to the 1847 birth.
The problems are that (i) the Wm Davidson (Snr) whose mother was Barbara Tawse is listed in the 1851 census at Sunnyside, being 38, and born at Turriff, and has a son Wm who is 4 and born at King Edward - this also ties in with the 1847 birth and as you say means the Sunnyside family is a strong contender, however (ii) the only 1847 KE birth I can find reads "17 April 1847 Wm Davidson in Mains of Craigston & Jane Barclay his wife had a son born, named Wm, witnessed by Alexander Barclay, Mains of Craigston - this possibly links to the Craigston family too.
Jane Barclay should have been born in Forgue around 1825. I have found a Janet Barclay born in Forgue 10.6.1822, whose parents are Alex'r Barlcay and Margaret Davidson. This does suggest the Forgue Barclays are right as you say above. The 1871, 1881 and 1891 censuses for Craigston show that Jane Barclay as being born at KE in 1824, and has a husband George Barclay so those Barclays are a red herring.
The Forgue Jane however is different to the Jane who died in 1895 in Macduff, listed as widow of William Davidson, Farm Labourer, whose parents were Alexander Barclay, Farmer, deceased, and Jane Barclay, ms Pittendreich, deceased, which is were I started out n this thread.
I'm now thoroughly confused and need a wee lie doon!