I can't imagine why you need Sheila to have "worked in super fast time." She was in an isolated farmhouse where presumably, no one was anticipating what would happen. She could have done the whole thing at her leisure. We know she wasn't always on the best of terms with June. She "could" bearing in mind that this is only a made up story- have shot June whilst Nevill was using the downstairs shower, THEN realized she'd have to shoot him, too. You can't cause Jeremy's guilt by unknown time scales.
Yes that's a good point, the children and June could have already been murdered by the time Neville got off the phone.
Plus what time did the police and Jeremy actually get to the house, they parked at the end of the lane which quite a distance away, did they have a chat for a while? They also returned to the car later to call backup.
Sheila, having been seen by Bews and Jeremy through the bedroom window, could have killed herself when they returned to the police car.
So I'm going to add another 20 minutes to my time frame. I've now got actions taking a minimum of 15-22 minutes carried out in a 58minute window.