I hate to throw a spanner in the works but you said in your opening post that William Prosser was born in Trevethin. Is this not more likely to be him with parents thomas & Margaret (there is another William Prosser born 1865 in Chepstow)
1881
Thomas PROSSER 51 Crickhowell, Brecknock, Wales B Smith
Margaret PROSSER 37 Llandenny, Monmouth, England
Thomas PROSSER 17 Trevethin, Monmouth, England
William PROSSER 15 Trevethin, Monmouth, England
Ellen PROSSER 12 Newport, Monmouth, England Scholar
Census Place Trevethin, Monmouth, Wales
Do you have William & Minnie's marriage certificate to prove William's father's name. There are a lot of Prossers in the Abergavenny area. I have some in my family too
there is also a William b1870 in Trevethin parents William & Mary