Can I just put forward a couple of things that might be happening here.
first we have to remember that while we want our ancestors to be honest and truthful they weren't. They lied about things, they manipulated the truth, they let other people assume things so
They lived as a married couple there could be any number of reasons why they did not marry immediately from one of them being already married to waiting until a relative had died and left them the fortune in the will, whatever reason, they lived together but let people assume they were married. They would not tell the children this because children have to tell their best friend the biggest secret so the lids wouldn't know.
The information on the children's birth certs is what the informant told the registrar - Try to find a death certificate for Elizabeth and see who the informant is - and where she died.
The Clutton marriage could have been because they wanted to keep it quiet and so the nipped back there did the deed and came back to Ebbw Vale, the trains were available then.
Just my four pennorth from years of chasing some really good lying actors
Ann