The Highlander's support from the Jacobites (particularly the '45s) did create a big backlash against Highlander culture (and consequently, their language), with stuff like the disbanding of the clan system, even outlawing the kilt in the 1746's Dress Act, that was the beginning of the end.
The later one was during the 1800s, when Scotland passed the Education Act that made English the sole language of education, it for example caused the percentage of Gaelic-speaking in Inverness to plummet from around 100% in 1800 to 4% today.
So you need either no Jacobites or less association with the Highlanders (though the British government didn't had a good opinion on the highlanders, which was one reason they went for the Jacobites) and a British government that is less interested in Scottish affairs with education being more of a local/private matter, though it may still decline/die out as result of (eventual) mass education and mass media.
Dunno about norn.