For Jeremy's plan to succeed he knows he can't let his greatest threat,Ralph(Nevill) near a telephone. Jeremy has left the anschutz rifle on the settle and this is his murder weapon,and also acts as the perfect excuse if by chance anything does go wrong Jeremy can claim that he came back into the house to put the rifle away and would have to postpone his murder attempt. However as we know in Jeremy's mind it had all gone swimmingly,so much so that by the September he didn't even see Julie as a threat which is why he felt able to two-time her with Virginia.
You still have not dealt with how the rifle which WPC Julia Jeapes saw resting against the bedroom window, some 15 minutes before the raid team set off to get into the house, ended up on Sheila's body in time for PC Bird to photograph it there, after 10am? Of course, if the rifle in question is the murder weapon, then it kind of puts your theory that Jeremy was / is the killer to the torch, so to speak. Anything that Jeremy might have done with the rifle beforehand, or as you say on the previous evening, has no relevance to who shot and killed Sheila with that rifle if she was shot under the chin by that rifle after the raid team entered the premises. You cannot get out of answering this matter by simply arguing that you do not believe or accept that the rifle was seen at the bedroom window by WPC Jeapes, or that you do not believe that the rifle was moved from the bedroom window onto the body after the raid team entered the premises - because all the evidence points to it being the rifle there, and that police moved it onto Sheila's body after photo' 23 (showing a view of the rifle resting against the window from the vantage point of the middle landing of the main stairs) was taken by PC Bird ...
The Mugford evidence you are trying to fill everyone's head with is nonsense and does not fit in with the now known facts...