I can see that you are clearly better at all of this than I am. At this point I am 1 for 4.
When I search SP for William McKay deaths the closest I get is an 83 yr old in 1903, in Marnoch. Only one year off in age from what you said, but I trust your hunting skills more than my own. In fact, when I search all William McKay deaths in all of Scotland in 1903, I get just this one in Marnoch. I don’t find any Williams in Cromarty in any year.
I did find Margaret’s 1908 death record in SP. I haven’t bought credits yet to download, because I want to see how many total I might need.
For the 1901 Census I do find a William McKay in Ross and Cromarty in SP, but he is 80, not 79. If you think that is close enough I will download it, but, again, I trust your skills more than mine.
In SP I don’t find any Elizabeth McKays dying in all of Scotland in 1935 even close to the age of 78. And none in Inverness. When I look at Inverness specifically I find an 82 yr old dying in 1930 and a 74 yr old dying in 1945.