I'm sure there would have been quite a commotion during your scenario - so what are June and Sheila doing at this point? And wouldn't it involve Neville making his way past the gunman in order to leave the bedroom?
I agree with this,that Nevill would have to run past the shooter to escape the bedroom. I don't see this as likely. My scenario is that Sheila was up and about and that Nevill went downstairs. Possibly Sheila had hold of the gun and locked herself in the downstairs toilet and Nevill was afraid she would harm herself. He may have called JB either wanting assistance to break down the toilet door or needing someone to protect June and the twins whilst he dealt with Sheila.Whilst on the phone,Sheila may have run upstairs which caused Nevill to end the phonecall,accidently knocking the receiver back off the phone in his haste to go after her. Sheila has begun shooting at June,Nevill appears in the doorway to the bedroom and is also shot. He runs downstairs in order to distract Sheila (or go for help). Perhaps he thought June would make it to the office phone? (which,funnily enough WASN'T hidden by JB)!!