Re: Mac vs. Mc, it is not nearly absolute, but much more commonly divided as I suggest.
And the McDonalds who founded the restaurant that the chain grew from, again, were of Irish, as opposed to Scottish, descent.
When it comes to descent like that, it’s tricky. Ever heard of the Scotch-Irish? All Irish MacDonalds/McDonalds come from the Scottish clan, who moved to Ulster initially.
And in the Gaelic language (both Irish and Scottish forms), there is no abbreviation. Everything reads as “Mac”, as in “Mac Dhòmhnaill” (Scottish form; Irish form uses an acute accent over the “o”).