Have you tried salt? Salt and iron... not making it obvious of course but Miffy could've smashed open your car trying to get to you, could've tried apparating into the back seat, could've but chose the "easier" option of waiting for you at your home. Why? You could try slipping some holy water into its dish, too. As for momma cat, while I hate to suggest it, is she a normal cat? It may be your responsibility to stop those kittens from being born :/ imagine a litter of Miffys unleashed on the world. It only takes a couple generations of ferals to destroy an ecosystem, imagine what hellcats might do to humanity.