Edward and Maria did have an "Uncle William" and is the one mentioned at the start of this thread. He was bapt. 6 Oct 1791 at Nottingham St Mary's, and died 15 Nov 1828. But, as you say, the William in the letters could be a maternal uncle, but I've never identified Mary Hardy WRIGHT's parents or family. I have her death certificate which unfortunately is severely limited in its information — oh for Aust/NZ certificates!
One of Agnes MILLLIN's brothers was Robert BROTHERS, b. 1865 Sofala, wasn't he? And he married Elma Editlh LILLAS in Sydney 1893? I can see two of their children in the NSW BDMs, and from a tiny Ancestry tree there is a son ([coincidentally?] with the same name as a Nottingham relative) … and they all ended up in South Africa! I have tried to contact the owner of the tree, but she has never responded.
I also thought it extraordinary that from 10 children — well, from the eight who survived to adulthood, Agnes MILLIN was the only descendant I could find. Mind you, Edward Wright BROTHERS had nine children who all married, so he may have made up the shortfall, as it were.
I shall attempt to send you a private message. If you don't receive it, reply to this post and I'll try again.
Koromo