Ah yes. Not only is this a pathetic attempt to make your scenario fit. It would also mean Jeremy knew all along that there was evidence to negate the silencer that could be presented to the jury and appellate court yet never told his trial counsel about it and sat in prison for 25 years before bringing it up. 
Let's think about this:
My own view is that Jeremy would not have taken the rifle back downstairs at all. He had no reason to, and he was also unlikely to, because remember that in order to figure out how to stage Sheila's suicide, he had to take the silencer off the rifle and then place the rifle on Sheila's body. He would have just left the rifle on or by the body and put the silencer in the gun cupboard before leaving.
However, we can't rule out completely the idea that after placing the rifle on Sheila's body, he decided that he needed to check on Nevill by prodding him with the rifle, so returned downstairs with the rifle having the intention of putting it back on Sheila's body later. So let's assume that Jeremy returns the silencer to the gun cupboard, and in doing so, he also checks Nevill's body with the rifle.
The immediate problem Jeremy would have is that Nevill's body blocks the way to the den. How does he get to the gun cupboard if he can't get through the back door of the kitchen into the back corridor?
It must be that Jeremy moved the body either before or after leaving the silencer in the gun cupboard (and if before, that means he's moved it twice). But why would Jeremy block the back door of the kitchen with Nevill's body? One possibility is that Nevill was still alive and after killing Sheila, Jeremy heard Nevill downstairs or even when he was in Nevill's den, and that is when the struggle between them ensued in which Jeremy hits a dying Nevill with the rifle butt, etc.
The next problem is how Jeremy can go through the back corridor and across the floor of the den without leaving blood traces and blood prints (feet, hands, fingers) on the floor and walls and in the gun cupboard itself. He is carrying a silencer that has blood in it and he must have blood on his shoes/boots and clothing as he has presumably struggled with Nevill. This I can't explain. Did Jeremy remove all his clothing and tip-toe across?