When reading Mr. Boutflour's statement the words that stood out for me were where he stated, "I find it inconceivable that in passing through the kitchen and hallway near the back door Uncle Nevill would have left a gun and ammunition lying around. Without doubt he would have put them away in the gun cupboard if he had seen them".
Now if he had taken the dog for a walk as was his practice then he probably would have seen them and with the added reason that the children were in the house he would no doubt have been doubly vigilant. (To my mind so should Jeremy. On his own admission he left a gun with a magazine full of bullets just lying around).
However, If Nevill had put the gun away and I believe we should assume that he did. Then the person who did the shooting had to go to the gun cupboard (which was unsecured) to get the gun and quite possible assemble it with the magazine, if Nevill had disarmed it properly. That I think should rule out any third party. The shooter was either Sheila or Jeremy. But which one?
Yes that's a fair analysis in my opinion.
As an 'anti' (

), the view that I have, is that the whole rabbit hunting incident never occurred, the rifle was not handled by Jeremy the night before, he did not leave a loaded magazine and rifle on the settle or dining table, I believe that story was fabricated to leave the rifle in an accessible location to give the impression that Sheila could easily get hold of it.
The removal of the sight is a bit of a weak point, yes there is the explanation from Jeremy that it didn't fit in the case (or gun cupboard, depending on who you talk to) with it on, but there has been no corroboration of that being true and it doesn't really make sense that the sight would need to be zeroed in each time.
(that's just my view on it)