Nevill would have quickly disarmed Sheila. If in the very unlikely 3am event that she was bare footed, in a nightie & holding a rifle for shooting rabbits. As head of the family he had no choice.
As the only adult male in the house, he could control the 4 others simultaneously if he needed to. He would quickly disarm Sheila, like taking candy from a baby. However there was a very slim chance he would be hit by a torso shot which would not deter or stop him.
Nevill would only have a problem if Sheila was holding a shot gun. Being hit from a shot gun shot would kill Nevill. This would leave the killer free to fire bullets into the other household members & commit suicide.
The sound of the shot gun may have woken June, Daniel & Nicholas and they may have got out of bed. Although it would be surprising if the raised voices downstairs had not already woken them.
There was no possibility Nevill would ever have rung Bamber. So whoever gained access to WHF in the morning would have discovered the bodies.
As everyone knows, the murder weapon was a rifle for shooting rabbits. Nevill would instantly take up the 100% certainly of taking the rifle back. Despite the very slim chance of getting a torso shot in the process.
Wonder if Bamber regrets the shooting rabbits story.