Personally believe he did it in his farm overalls. I don't believe he used a mask or gloves.
He didn't expect to be grappling with Nevill. He expected a quick massacre.
Having said to me to support his story of having Nevill phoned him he would need to have Nevill up and about.
Ending Nevill in bed doesn't support the theory of Shelia going ape shit to me.
Ideally Nevill gets shot in bed, gets out of bed, staggers a few feet, then collapses.
Or he can kill Nevill in bed then drag him a few feet.
He can then take the bedroom phone off the hook.
Either way he would not want blood on Nevill's side of the bed as it suggests he was ambushed in bed.
All tricky.