Have you found William McKay's death certificate? It should tell you the names of both his parents.
There's a death of Margaret Cobban, other surname McKay, aged 90, in Cromarty in 1908. Go to www.scotlandspeople.gov.uk, invest in a few credits, and use a few of them to download the certificate. That should confirm the names of Margaret's parents and the name and occupation of her husband.
The 1901 census (a transcription) shows William McKay, 79, steamboat agent, wife Margaret, 83 and daughter Elizabeth, 44, born Alves, living in Cromarty. You can get the original of the census from Scotland's People.
There is a death of William McKay, 82, in Cromarty in 1903. This looks like your William. Once again, get his death certificate from Scotland's People and it should tell you the names of his parents.
You were wise not to subscribe to Ancestry, as you would not have found any of this information there.