Maggie, I know only too well how very easy it is to spend heaps more than one intends on SP!
Perhaps (on the Dumfriesshire board as you suggest - with a link to this thread) if - once your cold has cleared up - you post all you know about your ancestor Christopher and your links to him (including the main points of the Will you've just downloaded?), someone might have some ideas which will help with your puzzle. I tend to be attracted to deciphering (including old handwriting) threads and I like puzzles so will certainly be looking for this one. If I don't post on any such new thread, it won't mean that I haven't tried - it will be because I've tried but believe that I don't have anything useful to contribute.

Best of luck!
Gatacre, The mother's maiden name is not in the online IGI for Christopher NICHOLSON, 1781, Mouswald for one reason and one reason only - because this is one of those infuriating online IGI batches where only the girls were entered so Christopher isn't there at all. If the boys had also been included in the online IGI for Mouswald, his mother's name (her full name - given name & maiden surname) would have been shown simply because it
was recorded in the Mouswald OPR.
John NICHOLSON, 1787, also doesn't show in the online Torthorwald IGI. Again because it too is a girls-only batch. But, IF the boys had also been shown in the online IGI, the mother's name wouldn't be there - simply because the mother was not recorded at all (not even her given name) in the Torthorwald OPR.
But you are, of course, entirely right about the persistence of women's maiden names in Scotland (luckily for genealogists) - often women who are obviously married (though relationships weren't shown in 1841) are recorded only by maiden name in the 1841 census, and quite often in later censuses women seem to revert to their maiden name after death of a husband ...
Incidentally, Roucan was identified on the first page of this thread (by moi) and subsequently confirmed; there's really no remaining doubt

Regards to all,
JAP