This is going to need an awful lot of research!
It is bedtime now, but if anyone wants to take on the baton til tomorrow, here is the 1891 census you mention:
RG12/4431/94/34
Amos ROGERS H M 29 - coal miner - Monmouth Llanithel English spoken
Ann W M 39 - do Bedwellty Both spoken
Annie PROSSER dau U 18 - gen servant dom - Glamorgan Gelligaer English
Edward H PROSSER son 17 - coal miner - do English
Francis PROSSER son 14 - do - Lancashire Aspoole English
Margaret PROSSER dau 11 - scholar - do Goldbourne English
William D? PROSSER 5 Monmouth Mynyddislwyn English
David W ROGERS son 2 - do do
Address: 20 Railway Terrace, Tir Phil, Gelligaer, Glamorgan
This certainly suggests that Ann was married to a PROSSER before marrying Amos.
I wonder whether the enumerator simply got the relationship with Henry WILLIAMS wrong? It might be very difficult to prove anything. Have you looked for Henry's birth (and considered investing in that cert if you have found it)?
kind regards, Arranroots
