I can't find any Inverness burgh marriage after 1750 which might be credible -there's an Alexander Mackintosh for whom Scotlandspeople claims there is no wife registered, but when you look at it, there is just a little cross, which might mean, I suppose, that no marriage took place, and he is described as being "in Essich" which is some way out. Nor is there any Ewen in the baptismal registers, but I didn't try all the possible spellings of Ewen. I will try again for marriages before 1750.
The document is very interesting in fact. If you follow it back into the Cambridgeshire catalogue, it appears to relate to the appointment of a Rev James Mackintosh to the Cambridgeshire parish in question who had previously been a vicar in Dominica - unfortunately the way these postings work i don't seem to be able to refer back to it without losing my draft, so apologies for any errors. I THINK it implies Alexander may have been present at his installation, but I may be mistaken. My guess would be that the document of the 1780s might represent some sort of financial arrangement that Alexander Mackintosh might have backed, though whether he was making a loan on the security of the advowson to two crafty legal men ( or maybe one, since the underage one was probably the patron), or simply guaranteeing security I don't honestly know. £950 is a fair chunk of credit. But it may well be that James Mackintosh was a kinsman of Alexander's. It looks a bit murky, but the twenty-first century has nothing to teach the eighteenth about squeezing every bit out of every possible asset. I think, on the strength of the size of the amount in this reference and the Clarks' connection with Coutts, it might be well worth asking them whether Alexander had an account with them. They do keep what they have. The worst they can say is "No".
I am honestly coming to believe that no-one at all in late eighteenth century Inverness was free of some sort of transatlantic connection to the slave economy. The document's clearly worth following up but I don't know where to start looking for James Mackintosh. The Bishop of London's papers in the Guildhall archives (or the London Metropolitan archives or Lambeth Palace) might have something about the Dominican end, but the London diocese, which ran the colonies, doesn't seem to have been very interested in ancestry per se, and you have to go along and search, since their online catalogues simply list the collections.. However, there's a wonderful grab-bag of Caribbean material called Caribbeana - a turn of the (twentieth century) genealogical periodical which used to be extremely rare, and has now appeared online from the University of Florida and doesn't hide behind a paywall. I'll have a look and see if he turns up in Dominica, though it tends to be the bigger islands which crop up more frequently.
I don't have ( or can't find) here my note of the Golden Square address, except that it was in Sherrard Street. The Cambridgeshire document has him in Haymarket so that puts it back before 1785. I think the source might be one of the burgh documents and my notes on them are in Inverness. Did St Martin's cover Golden Square, or might it have been St Anne's Soho? Sometimes it's worth ringing up Westminster Local History library (if they haven't closed it and sold it off), or their archives. They seem to have a fair spread of directories for late eighteenth and early nineteenth century London.