All he proved was that Nevill was shot four times upstairs and then finished off downstairs following a struggle. He had nothing to say about where Nevill was before the shooting started.
It wasn’t Vanezis’ job to determine where Nevill was moments prior to the shootings; his job was to prove how Nevill died
What you’re trying to infer is that Nevill phoned Jeremy to come and rescue them all from Sheila — and that is a ludicrous suggestion
Had Sheila suddenly gone berserk and was waving the gun around upstairs that would have been a dire emergency. Nevill wouldn’t have had time, nor even would have thought to leave his family in grave danger by going downstairs to phone Jeremy. That would be akin to watching your family drowning and instead of jumping in to rescue them, trotting off to a telephone and calling someone who lived three miles away to come and save them...and then for good measure, calling the police 20 miles away (after looking their number up in a directory) and telling them too that his wife and grandchildren were currently drowning
TIME would have been of the essence, and in such an emergency your natural reaction would be to save your family
Nevill would have — and could have — disarmed Sheila quickly by simply throwing himself on top of her. According to Jeremy when he himself phoned the police, Sheila was just “going berserk” with a gun — he never said that Nevill told him she’d shot anyone — so that would mean Nevill left Sheila alone up stairs “going crazy”. He wouldn’t have left her free to kill his family!
You’re also forgetting that had that been true, and no shots had been fired, Nevill possibly wouldn’t have even known if the gun was loaded — so all the more reason he’d have swiftly disarmed her. But whether it was loaded or not he’d have still taken swift immediate action.
And after disarming her if he was going to phone anyone it would have been the local doctor to come out and sedate her...