I'm not a Scots speaker and none of my family are, but Scots is not a dialect of Modern English. Modern English and Scots are both descended from Middle English, albeit with different influences, and are both members of the Anglo-Frisian branch of West Germanic languages. The reason that most Scots speakers do not view it as a language is because it exists in a continuum with Modern English in modern Scottish speaking patterns, by necessity and for historical reasons. You try comparing Scots in its full state against its Modern English sibling for most of their shared history and they are mutually incomprehensible except when the Scots speaker uses Modern English words. The reason that non-Scots speakers can read Burns' poems is because it is light Scots, intermixed with standard English words. There are no set definitions in linguistics for dialect vs language, not that aren't argued about, but for me personally I can't view things that are mutually incomprehensible in their full form as being dialects of the same language. Venetian and Sardinian are not dialects of Standard Italian, Portuguese is not a dialect of Castilian Spanish and neither are Aragonese, Leonese, or Catalan, and Scots is not a dialect of Modern English. Scottish Vernacular English is a dialect of Modern English, which often utilises Scots words regarded as nonstandard and slang by people judging it against standardised British English, but that is not the same thing.
I'm also suspecting you are not very familiar with languages in the British isles, there not being such a language as 'Scotish', and because you seem to be unfamiliar that Scots was the courtly and legal language of Scotland under the Stuarts before personal union with England. If by Scotish you meant Gaelic or Gallic then no, it would not have been the official language of Scotland, the Stuarts and prior dynasties explicitly undertook campaigns to 'civilize' the Gaelic speaking parts of the Scottish Kingdom. There is a reason why the language is called 'Scots', because that's the equivalent of 'Scottish' in dialects of northern England and Scotland. They already considered it different from English as a language, the same way that Old English speakers distinguished their language from the closely related and highly similar languages of Frankish, Old Dutch, Old Frisian and Old Saxon.