Hi Bitza,
My family were/are Morrisons from Lerin (near Durness) and it seems that about every second Morrison married a MacKay. Consequently, I have had an interest in the MacKays for many years.
There are two very interesting, and useful, books that I suggest you try to find. They are:
"History of the House and Clan of Mackay" by Robert Mackay of Thurso; and
"The Book of Mackay" by Angus Mackay.
I believe both are now out of print and your best bet for gaining access to them is a large public library or a university library. Both contain narratives of the clan history and genealogical accounts, i.e. that of the chiefs' line and the main branches of that line, ( the clan proper). I believe there are more people with the name of Mackay who are not related to the chiefly line than those who are.
The origin of the Mackay chiefs seems obscure and "authorities" cite different sources. Malcolm MacHeth (or MacEth) is often mentioned and clearly he was of royal blood. King Malcom MacKenneth is also mentioned frequently but I do not know of any source that is undisputed.
If you know which branch "your" Mackays are from, I could look in the forementioned books to see if your known ancestors are mentioned. You appear to have traced them as far back as 1700 and these books give fairly good detail from 1700 back to 1210.
Ian