Your arguments are silly. Two can play at that game However
Could Neville have put the gun back in the cupboard? YES
Did Sheila know there was guns on the farm? YES
Did Sheila know were the gun cupboard was? YES
Did the gun cupboard have a lock? NO
Had Sheila held a gun before? YES
Does this mean Sheila know one end of the gun from the other? YES

I said this week on another thread that Bamber was trying too hard to set the scene pre massacre to frame Sheila. Telling everyone there was a fully loaded rifle sprawled across the kitchen for her.
He invented a batch of very unlikely events which he has always stood by -
Seeing rabbits.
Deciding to go outside and shoot them.
Fully loading the rifle.
Not putting the silencer on.
Not firing any shots at rabbits outside.
Not putting the rifle back in the cupboard.
No one else putting the rifle away into the cupboard.
Fostering conversations were apparently taking place during this period.
He could have just said there were guns in the gun cupboard and Sheila must have taken one. He had already lied to the police and said Sheila knew how to use all the guns. Maybe he felt someone in a psychotic frenzy would not have the presence of mind to go to the gun cupboard.
Anyway the rifle on top of Sheila was the same one Bamber said he left out. Once he had given the story of leaving the rifle out and implied Sheila took it, there was no going back from the shooting rabbits scenario.