If Nevill sustained those facial injuries in the bedroom he'd have been incapable of making a phone call.
If the theory is correct then i think Neville must have been unharmed when he made the call ...
It is possible he was marched at gunpoint back upstairs to his bedroom after the call ...
He may have first been shot in the bedroom
He may have escaped down the stairs ...
He may have been trying to get to one of his own guns from downstairs ...
The shooter may have caught up with him in the kitchen and finished him off ...
Why is this not possible or far fetched ?