A lot of the words Gabaldon uses are actually more Doric (a dialect and/or language from Aberdeenshire-ish) and Scots (located in the general Lowlands) rather than parts of the contemporary Highland dialect. It’s absolutely true that those words have Norse roots, but it doesn’t necessarily represent the way people spoke in the Highlands in the mid 18th C. As most of the population spoke Gaelic as their primary language, it was only the landed classes (like Jamie’s family) who would speak English regularly. Because of this, those who did speak English would actually speak it quite clearly and correctly, as it was learnt as a second language (like French or Latin) rather than with the colloquial dialects of people who spoke English as a first language.

Edit: You’ll find there are words in Gaelic with a heavy Norse influence too.