djct59 - excellent and complicated questions!
It is a bit before dawn here in Northern California, and a work day for me, so I'll get started on some details, but will likely have to stop before I get all the pieces in place.
Let's start with the dates of birth and death for Kenneth MacCulloch. I do not have solid source citations for either, so I use working placeholder values that are circumstantial.
His wife Barbara Ross has a clear baptism in the Durness Parish Register (DPR) in 1768, so that's not a placeholder value. Kenneth does not have a baptism, therefore I assume he was born before November 1764.
Kenneth and Barbara have a marriage registration in DPR on 5 February 1788. This is followed by 7 baptisms of MacCulloch children in the DPR (James, Marion, Hugh, John, Roderick, Joanna, and William), but then there is a fascinating baptism in Thurso, Caithness, image attached. This is an open area of investigation that raises many questions, but for simplicity I let its details stand at face value. Here's how I transcribe the Thurso baptism of their son James MacCulloch: "1804 April 8 - James l. s. [legal son] to Kinneth McCulloch Day Labourer in Town & Barbara Ross Wit: [witnesses] Hugh Morrison & Patrick Dun".
Patrick Dunn is throughout the DPR as an incomer to the parish from Peterhead who married Janet Mackay and together they brought 1 marriage and 8 baptisms into the DPR.
Patrick Brown a single Man in Keoldale had a child baptized called Ann 24th November 1775 but not in lawful Marriage.
Patrick Brown from Peterhead in Keoldale married Janet Mackay alias nin Neil in Keoldale, 4 March 1777.
Patrick Brown from Peterhead, a little tenent in Buailavig, Keoldale, James, 8 October 1777.
Patrick Dunn, alias Brown, tenent in Buailbhig of Keoldale, Jean, 10 January 1779.
Peter Dunn, formerly Patrick Brown, tenent in Keoldale, and his wife Jannet Mackay, alias nin Neil, Neil, 16 March 1780.
Patrick Dunn, alias Brown, tenent in Grudie, and his wife Janet Mackay, alias nin Neil, Jean, 6 May 1782.
Patrick Dunn, alias Brown, tenent in Grudie, and his wife Janet Mackay, alias nin neilicemich, Fairly, 25 November 1784.
Peter Brown, alias Dunn (from Peterhead), tenent in Grudie, and his wife Ann Mackay, alias nin Neil, Peter, 20 February 1787.
Peter Dunn, alias Brown, tenant in Achairn and his wife Ann Mackay, alias nin Neilmacemish, Margaret, 6 April 1789.
That's Peter or Patrick Dunn or Brown and Janet or Ann Mackay nin Neil (with variations). Seems a bit of a rascal with the meandering names, but I suspect he was an endearing companion with a dependable gift for improvisational word play. I've traced a few but not all of their kids. Their oldest Ann Dunn is relevant here because she married Donald Ross (1771-1862) a younger brother of Barbara Ross and an older brother of John Ross (1775-1857) my 3rd GGF.
Returning to the MacCulloch thread in Thurso, baptism witness Patrick Dunn was likely also a traveling companion day laborer in Thurso from Durness as the father-in-law of Barbara's brother Donald Ross. The DPR locates these folks in Keoldale, Grudie, Crosple, Auchchoran, etc. so it seems coherent.
Hugh Morrison I assume to be Kenneth MacCulloch's brother-in-law, married to Mary MacCulloch. This couple are also the parents of the Hugh Morrison who later married his cousin Joanna MacCulloch ("1818 - Oct. 7 - At the Manse Hugh Morrison Shep'd Glassbhinn and Johana McCulloch Saingobeg.")
At this point I have to pause, leave for work, and continue the thread later.
Regarding day labourers from Durness in Thurso civil and parish records, this is the only one I've found, but I suspect there are more.