I haven’t quite pieced everything together yet, but I think you were on the right lines with Margaret Fish, Monica.
So, here’s what I have:
James Ross married
Janet Sloane on 6 Aug 1845 in Glasgow. As we know, Janet Sloan/Ross died in 1861, along with her infant son, Charles.
James Ross then marries
Margaret Fish in Edinburgh on 3 Sep 1861 - he is noted as aged 33, musician and a widower. Margaret is also noted as a widow, aged 24, a hawker of caps.
William Ross is born to James Ross and Margaret Fish in November 1862 in Glasgow. Margaret Fish/Ross died in 1863 in Glasgow.
I can’t obviously find a marriage for Margaret Fish prior to her marriage to James Ross, but she did have an illegitimate son -
George Fish - the birth was registered in Ardrossan, Ayrshire on 13 Sep 1858, by
Amelia Hislop, Margaret’s mother. The family lived in Saltcoats.
Amelia Hislop is the same
Amelia Wright that you’ve found with the infant
George Knox in Edinburgh in 1861. I think we have to assume that George’s father was a Knox, although I can’t find any evidence that Margaret ever married him.
Back to Amelia Hislop - on Margaret Fish’s marriage registration in 1861, her parents are listed as
John Fish, plasterer and
Amelia Fish ms Hislop. However, the plasterer was almost certainly
John Wright (as you’ve found on the 1841 census in Glasgow), Amelia’s second husband. Her first husband, and almost certainly Margaret’s father, was
George Fish, who she married in 1819 in Greenock, Renfrewshire.
An Amelia Wright (other names Hislop and Fish) died in 1869 in Langholm, Dumfriesshire.
As I say, I don’t think I’ve quite got all of the pieces together yet, but it looks like neither George Ross nor William Ross were Agnes Cumming’s children, both were Margaret Fish’s.
What a tangled web!

Ruth