Nevill was shot 4 times in the main bedroom.
He then went downstairs where there was a kitchen fight & he was killed.
Well I respectively disagree, the first shots to Nevil in your scenario are all over the place, torso, face etc. this strongly suggests Sheila. AP said he took part in shooting competitions and was pretty good with a gun and JB was as good as him.
Who ever was in the bedroom this is your scenario not mine, Nevil would not have just run past.