Looking again at that 1851 census, I'm wondering if the lodger, James Humings, is actually our James Homans ..........
It seems, from what we have seen, that maybe (and this is only a possibility) that the marriage to Coleman broke down. Sophia returns to parents, has affair with lodger. Eventually marries him.
Either George Coleman dies, or marriage is annulled for some reason, divorce is unlikely though possible, or second marriage is bigamous.
Another less dramatis scenario is that Sophia is just temporarily with her parents at the time of 1851 census, George being off mining somewhere, they have child Sarah a few years later. George then dies, (or marriage annulled/divorce etc) Sophia remarries to her parents lodger, but for some reason calls herself a spinster - she is illiterate so would not have read what the vicar wrote on the marriage cert. However, James Homans and the Davis witnesses weren't, so if they were aware that the details were incorrect they should have said!
What is the exact wording on Sarah Janes birth cert about her parents?