Yes, I did some research on the Graham family involved some years ago, I think William Graham had the sympathy of the court, being a good looking young man (and attacking gamekeepers was a fairly common occurrence as well). But the Grahams were related to a few families in the area, so living in Ainstable after the trial must have been hard for Francis Boustead.
He and his wife appeared to have stayed together, and it is quite possible they had had an irregular marriage in Scotland, but the marriage registers from the Gretna "priests" don't have much information on, so its probably not worth the time spent trying to find it.
I've researched several familes who claimed to be married when they weren't, one had a spouse in an asylum: one had a violent husband and ran away with a lover and changed her name, and the other had a husband who was already married to someone else but separated. So claiming to be married when you are not is not always because you don't want to, you might not legally be able to get married again.
an interesting family to research though!