My point is, if he had gone downstairs to phone Jeremy, he said Sheila had a gun, so why he would leave his wife in bed alone, when his daughter has a gun seems odd. Logically you'd ahve thought he'd wake her up and take her with him if he'd heard something (Sheila with a gun). Unless June was shot first, whilst Neville was in bed next to her?
Your right, the bullet cases could mean nothing. Especially with scuffles etc, they could get kicked around the place.