Good. We agree that June would not stay in bed if Sheila was the killer. However disagree that June being shot with her head on the pillow means she was almost certainly asleep when Jeremy shot her.
If June was shot while sitting up in bed by Jeremy, wouldn't she fall back against the wall, still in a sitting position ?
My view is that they were both asleep when Jeremy entered the bedroom. People usually are at 2.30am. There is no reason why Jeremy would have woken them when quietly entering a loose downstairs window & then creeping upstairs.
Jeremy opened fire at an asleep June with her head on the pillow, with the silencer attached. This would only take a few seconds & he believed he would be able to then shoot and kill an asleep Neville, who was next to June in bed. But Neville woke so the shots at Neville by the bed were not fatal. Jeremy overestimating the rifle power.