Not surprised it's confusing!
Alastair is an anglicised spelling of the Gaelic version of Alexander, which might remove some of the confusion.
You need to look at the originals of these various baptisms on the Scotland's People web site, and see what additonal information they might contain - an occupation, or residence, or maybe even the names of witnesses. Margaret's baptism in 1717 might be especially helpful, because if she was the daughter of Donald 'Mor' Gunn who died in 1716, it will probably say that he was deceased.
The problem is that the further back you go, the more incomplete is the information available. In particular, many (most) people's baptisms are missing from the surviving records. So just because there is a possible candidate in the records doesn't automatically mean that (s)he is the right person.
You might be able to find estate records, for example rental records, that would shed some light. According to the Valuation Rolls the proprietor of Crosskirk in 1855 (the earliest available online) was James Sinclair. James Sinclair Esquire was still a significant landowner in 1875, but he does not appear in 1885, so must presumably have died. It looks as if he was James Sinclair of Forss, who died on 1 March 1876 at Forss House. He was a descendant of the Sinclair of Forss who would have made the tack of Crosskirk to the original Gunn there, whoever he was.
The National Records of Scotland
https://www.nrscotland.gov.uk/ has numerous references to Sinclair of Forss. There could also be uncatalogued material, for example sasines and deeds, that would provide additional information. For that, you'd probably need to get a professional searcher to look on your behalf unless you want to spend many days in the search room in Edinburgh.
It would also be worth investigating whether the Highland Archives
https://www.highlifehighland.com/archives-service/ has any material relating to the estates of Sinclair of Forss.
Also consider contacting (or even joining) the Caithness Family History Society
https://caithnessfhs.org.uk/ Hope this helps.