If it was anything other than his mother's conversion, I'd be right with you on Patrick not quite believing that people lived and breathed before he existed. But if his mother was a convert it strikes me he'd know and care about it...unless she's concealed it from him... Oh, can I still have my headcanon about Helena being the sole heiress of a substantial mercantile fortune (possibly Dissenting) which to the impotent dismay of her family she ploughed into Mariot Chase?

(I know this has been extensively discussed before, but although there are lots of possible and entirely orthodox explanations for Patrick's having no siblings, the one I like best is that the Merricks, like good English gentry, keep separate bedrooms*, and Helena infallibly requires a two-cigarette long break before Anthony joins her. Anthony knows full well why, and he regards it as a matter between his wife and her confessor. Patrick, however, twigs sometime in his late teens or early twenties, and is upset.)

*actually canon, no? Nicola changes in Mrs Merrick's bedroom in FL?