Morning Susan,I think it was Neville who'd " clipped " Sheila in the neck with that first bullet, while they were both in the kitchen. It would have been done while he was sitting,injured,in his chair for the bullet to have gone in the direction of Sheila's neck if she'd have had her head tilted sideways.
I've always maintained that at least the first shot wasn't self-inflicted,though there's every possibility that the second one was, once she gained enough strength to get upstairs from the kitchen to the bedroom.
The reason that I believe Sheila went upstairs was that EP on seeing two bodies on entry,failed to say two " dead " bodies,as Sheila could well have been alive at that point.Neither body was examined when police had entered,and because the police weren't familiar with the lay-out of the farmhouse they wouldn't have known about the stairs which led up into the bedrooms.
I'm sure we'll get to know in due course of how everything panned out.