Oh, it's not my theory and I'm quite happy to see it shot down.
I think the whole George Smith link is entirely tenuous, if he existed at all.
I had seen that 1881 census reference, I think it was early on in my search and I wasn't quite convinced for some reason. It does seem to be a reasonable match, and having become more familiar with the geography I now know Kincardine O'Neill is not that far from where the family lived.
The nephew is listed on the 1881 census as living with the grandmother Mary Murison as George Smith. I'm not even sure how it has been determined his name was George Smith Murison.