It seems there was a misunderstanding of the diagnosis on my nan's arm. It's not melanoma, it's a myeloma. It is currently a single-site cancer, and all the tests they ran when they thought it was melanoma indicate that it has not spread, but it appears to be a Stage II myeloma judging from what I've been told and the information on this site.
It's still not good . . .
"Without treatment, the average survival time for multiple myeloma is only eighteen months. Younger patients are more likely to respond to aggressive treatment, which may improve survival by as much as four to six years."
. . . but it's a better prognosis than it was.