How did he end up downstairs then? Explain the scenario to us: Nevill wakes up to find Sheila waving a gun about but not having shot anyone and then what?
We are told by JB he received a phone call from NB:
"Sheila's gone crazy she's got the gun".
We could all speculate about exactly what was or wasn't going on. All that can ever be determined is what the physical evidence supports: blood stains, casings, distance of shots, trajectories of shots and wound tracks. When all these pieces are carefully fitted together they complete a picture or video on par with cctv.
The above clearly shows NB sustained his facial gunshot wounds when he was stood on the landing stairs and the perp was inside the bedroom shooting out onto the landing. NB then turned to retreat downstairs and when he was on the main staircase he sustained the two gunshot wounds to his rear.
It is not something you can arrive at overnight. You need to work through the pathology reports, blood stain evidence, casing evidence, ballistics evidence re distance of shots, layout of the farmhouse.