One thing I have discovered over time reading "The Steel Bonnets", a book about the Border Reivers, is that much of what was written down at least during those times was done so phonetically, sometimes with the same author using multiple different spellings for the same word in the same document. These were predominantly documents written by government wardens on both sides of the border---scot and english--but both followed similar phonetic styles. I get the feeling that formal spellings for most of the english (and probably scots) language were not standardised at that time or for some centuries to come, as even some documents from the kings during these torrid centuries had similar phonetic spellings. From what I have seen Scots written language is a combination of the local dialect with this phonetic based word forming technique, which may or may not have ever been formalised. English spelling at least has obviously been formalised as is witnessed by the plethora of dictionaries.

Gaelic seems to be totally different from Scots, although I could see how outlanders could confuse one for the other, especially given the large variation in dialects trying to speak the same language over such a relatively small country.