I think the mother only used one wish, the other coffings were used by her brothers or relatives when they searched for power and such. She only wished for a boy, but like Annette (or Giovinetta idk the name changes) said at the start, the wish can't be too ambiguous or this things happen. The wishes are doomed in any case since it's a devil that grants them.
John probably has two sides to him, one that's the devil and one that's himself that slightly knows what he does in his true form. He needs to blend in and copy humans so I think it was a way of survival rather than another wish
I guessed the mother used more then one wish because u see from the flashback there were more tombs unbroken compared to the present where only one is left intact.
And hmmmmmm idk but I think Annette's wish unsealed his demon side, since maybe I'm overthinking it but his behavior is changing
So........
John is a demon in disguise and the mother used all but one of the coffins but what did she wish for besides birthing a son and summoning the Demon?
WAIT did the mother wish for John not to remember he was a demon in disguise and live and act like a human. But only in like intense emotional situations does his demon self come to surface, like Mc getting married.
He DEFINITELY killed her OG Husband out of murderous jealousy.
I'm so invested into this story