I understand that 90% of your concerns rest on orthographic practices in the modern English speaking world rather than medieval culture. Essentially what you're after is names for guys on royal demesne ... bulk of counts were Gaels for the bulk of the CK period, including most of the ones with "non-Gaelic" names. What you're after is essentially a culture tag for the king, the count of Berwick and the 1337 counts of Angus and Moray, all of whom came from the area represented by modern Lothian and Borders region. If you want to push the lack of culture tags, the most sensible thing would be not to have any tag representing "Lowland Scots" (who didn't exist in 1187 or 1337 ... two scenarios!), and use it usefully for Gall-Gaels or Scottish Gaels.
This is jordar's mod. He can name it what he likes, but so long as I'm here I will give him the advice I consider sensible. Your idea for the tag isn't sensible IMHO, and I say that both as having experience remodding the CK Scotland set-up and culture set ups, and as someone with two history degrees, including a research degree in medieval Scotland. Ignoring the offensive side, which obviously isn't important, would it help if I started quoting sources to you? This would really get across why your name suggestion might look so ridiculous to some.