I know that Norn, a Western Scandinavian language, was spoken in Scotland for quite awhile, before it's extinction.
Norn was spoken in the Orkney and in the Shetland, not in the rest of Scotland as far as I know. The old nordic language that was spoken in Scotland (where Gaelic is spoken now, and elsewhere) was Old Norse.
Could Scottish Gaelic's voiceless-only plosives and pre-aspiration be because of Western Scandinavian influence, or pure coincidence? Please give thoughtful answers, not just "yes" or "no". Looking forward to hearing everyone's thoughts!
Yes, it is because of the influence of Old Norse that Scottish Gaelic has preaspiration etc. Old Norse had influence on Scottish Gaelic in vocabulary too (and on grammar too, even on Irish and Manx grammar actually).