Manx is one of the Gaelic languages, so it’s a lot closer to Irish than to Breton, Cornish or Welsh. Its most unusual feature is its orthography, which was created in the 16th or 17th century by a Welshman who essentially wrote it down as it sounded to him - so it looks nothing like the other Gaelic languages. But in terms of vocabulary, pronunciation, syntax etc., it’s pretty similar.
Manx – along with Irish and Scottish Gaelic – is a part of the Goidelic (Gaelic) branch of the Celtic languages. Welsh, Cornish, and Breton are part of the Brythonic (British) branch.
Scots is a dialect, not a seperate language. Or at least that is what I got taught whilst studying for my English Language degree, in a Scottish university. It has a LOT of different words, but the sentence structure is the same, and what deviations it has are no more wayward than, say, speakers from the South-West of England. Of course it’s a fuzzy boundary between language and dialect, and the boundary often seems politically determined.
And I think Gaelic’s in a lot of trouble: it’s becoming pidginised, as every single Gaelic speaker in Scotland speaks English, and Gaelic is not exactly adaptive. However, Welsh looked in a lot of bother not so long ago, and that’s undergoing a remarkable revival, so who knows?
The terminology you’re using (deviations? wayward?) speaks volumes, but I think it’s debatable that the difference between Scots and “Standard English” is no greater. In any case, though, that doesn’t necessarily mean it isn’t a separate language. Serbian and Croatian essentially differ only in the written alphabet they use, and yet they are (now) considered separate languages. As you say, it is mostly a political distinction. There is certainly no logical justification for the definitive statement with which you opened your post.
Fair play - but the OP did seem to me to be asking if Scots was a distinct language to English. ‘Deviation’ is a fair word to use in describing the differences, if somewhat emotive. And trust me, I’m not blowing smoke up your arse on the latter point, Scots isn’t really any more distinct than other dialects the length and breadth of Britain. Of course, you could argue that other dialects are languages too.
I think you’ll find I instantly qualified my definitive statement - my logical justification being that the wonderful, dry world of linguistic academia deems it so. Of course, that doesn’t make it necessarily so, it’s subjective. But it’s a reasonably strong position to be sitting on in an argument.
We’re in GQ, ‘trust me’ isn’t a sufficient reply. How many other dialects in Britain have systematic syntactical differences, a different orthography, and a different historical evolution (see diagrams on this page ?). Serious question.
But nobody does argue that the other dialects are languages. I think that’s fairly significant.
I don’t know that your particular instructors at the university you attended speak for all in linguistic academia, though.
Spanish and Portuguese are separate languages, right? Once my wife took a Spanish literature class in college, taught entirely in Spanish. A Portuguese professor came in one day talking about a class she was teaching, and she spoke in Portuguese, but my wife had very little trouble understanding her. So the difference between “language” and “dialect” isn’t very clear sometimes.