That's the way I'm leaning, Groom. Alternative scenarios don't make a lot of sense. Furthermore I have searched a fair bit for evidence of a Ms Atkinson of the right age in the region and at the time, and have not found anyone.
Do you have any opinions about the birth of the first two children (William in 1847 and Ann in 1849) up to four years before James and Mary Atkinson tied the knot legally? This could simply be what it appears to be, but I wonder if instead there might be an earlier marriage cert for this couple that somehow was not considered good enough, or of the right denomination, for life in Cty Durham in the 1850s. In other words, recertifying an existing marriage.
-DC