Father's dad is doing his best to set up his "son"
I think Deckard might be born somewhere else, what matters is that the witch is trying to provide a body for her divine patron. after all, if FMC of the novel was born earlier than expected, then that witch might offer someone else as an vessel. and dad still might be killed just because.
I do hope to see Deckard and his redemption, but if theirs going to be a timeskip, it's going to be a long while before we see any of that. I also hope that our protagonist can see the FMC, spoiling her niece might relieve some of her stress