Do you not believe a fully fit Nevill would instantly reclaim the rifle if in either of these two situations -
Sheila had the rifle & looked to be a threat to his family, herself & him.
Sheila had the rifle but was not being threatening with it.
He had no other options & no obstacles.
By way of analogy and to help understand, let's imagine a young child has a rifle and is pointing it at a grown man. As it happens, this grown man has military training. Let's say he was in the Regular SAS and is a crack shot with a rifle, parachutes for fun and would kill you as good as look at you.
Now, is it a simple matter for this hardened SAS man to 'instantly reclaim' the rifle from the child?
Let's complexify the situation and say:
The child is having a violent tantrum.
The former SAS man is 61 and is tired after a day of manual work.
It's the early hours of the morning.
The child has her finger on the trigger.
The rifle is loaded.
What does our SAS hero do?
I do accept that it is possible he does, as you say, instantly just take the rifle from the child. But equally, you could argue that it's not that simple because if he moves towards the child, there is a chance that the rifle could go off and he gets shot.
Remember also that a child is small and nimble and can run around, maybe quite quickly, and therefore avoid being caught or make herself difficult to catch.
Of course, the SAS man knows all about weapons and let's say this is a small calibre rifle, so he may weigh up the risk and decide he should rush her. But a small calibre does not mean that the rifle is harmless. It could inflict death or serious injury, if fired. It could hit him in a vital area: a shot to the head, chest, groin or thigh could potentially kill him, or if not that, could do immense damage. A shot to the eye could blind him. That will be on the SAS man's mind too.
You guilters try to over-simplify the situation - which is not an encouraging sign. Don't misunderstand me: I am all for the simple approach, but if we're going to keep it simple, we could say, well, Sheila was found with the rifle and all the entry points to the house were secured from the inside. She was mentally-ill with a history of violence. Etc., etc., etc. Two can play at that game.
In my posts above, especially the first one, I've given you the alternative possibilities based on a presumption of innocence. These should be weighed in and considered and we need to ask: Are these reasonable possibilities? If they are, then potentially (subject to other considerations) the verdict could be Not Guilty. Personally I think these other possibilities are reasonable, which means there is doubt, which means we don't know and we have no right to keep somebody in prison on that basis, just because we think it more likely he did it. That's not good enough. Sorry.