Since Nevill was found dead and I believe Sheila then shot herself. That is enough for me to deduce that Nevill failed to disarm her.
You cannot take a gun off someone without being close to them. If someone threatend you with a gun, are you going to then advance towards them and just take it off them? You would have to be very lucky not to get shoot you on your way towards them. In such situation you would have to negotiate with the person holding the gun and hope to persuade them to put it down.
Nevill was not fully fit. He was in his 60s. The fact a gun used to kill rabbits broke the bone in his arm in half, makes me suspect he had some bone problems caused by old age.
Nevill was a fit farmer, 6.4 and 15 stone. He would over power Sheila easily. It's common sense that he would attempt this. Sooner rather than later as he needed some sleep.
He had no option but to do this. To negate the minimal danger of a light un uncordinated woman with no rifle experience, bare footed in a nightie holding a rifle for shooting rabbits.
Nevill would have easily disarmed Sheila. Either getting close enough to her while talking, or charging her in a small room. Maybe using an object to protect him while charging.