Personally, I don't think he had to get passed Jeremy. I think Jeremy came straight back to WHF after calling Julie (possibly on the pretence of apologising for an earlier argument over the rape seed load). I think he waited for Nevill to go in the shower (thus all adults are separated), shot the twins, then incapacitated June and Sheila and shot Nevill as he came up the stairs.
A very interesting theory. So you think that Nevill was still up when Jeremy came back. Wouldn't he have heard Jeremy getting into the house?
Then there's the issue of the cartridges in the bedroom. They were mostly grouped on the side of the bedroom nearest the door, but I suppose Nevill could have been shot as he got to the doorway from the landing.
I don't think he could have just incapacitated Sheila at that time. I think the two bullets fired at Sheila happened one after another, and I don't think Sheila would have been in the main bedroom at that time.