Thanks for hanging in there with me. Another thought.
Maybe he was an orphan, possibly some kind of relative who's parents died so he was christened with a different last name but had the first name of William. When they adopted him and brought him into the household they changed his last name to theirs?
Also when you pulled up that family that contained Reginald and Bridget and Bamfylde was that from Washfield?
Your other idea from yesterday that he used his middle name might explain it too.