Can't see what would wake Nevill. From a deep sleep behind closed doors.
Bamber had used two bullets on the twins. With silencer attached.
Nevill waking & investigating may mean he goes out of the bedroom. As well as wake June.
Not necessarily. Remember that, from our perspective, this all happens quite quickly. Nevill may well have been awake and still in bed wondering what the noise was.
I do see your point, though. If Nevill has heard something that is enough to make him wonder, then wouldn't he investigate? Yet what evidence is there that tells us he wasn't either just getting out of bed or even standing in the bedroom as Jeremy entered?
We've gone over all this before, of course, in August last year. Another issue is why it appears that Jeremy shoots June first (for some reason, I had come to that conclusion - I can't now remember why)?