An operator could listen in on any line, the handset doesn't need to be off the cradle for this to happen. Jeremy Bamber claimed that his father phoned him at his cottage and that the line went dead while he was speaking to him. Does anyone really believe for a minute that Sheila could have done this?
I don't believe a reliable reconstruction of the shooting in the bedroom could be accomplished since all we know is where Nevill was when he was shot and approximately where his killer was standing.
I should add to this the difference between listening in on an open line as against a closed line. If the telephone cradle was down because the handset was resting on it then the microphone on the handset would be disconnected. In that situation no sound could have been heard coming from the kitchen. If the cradle was raised as was the case at White House Farm then the microphone in the handset would be live, in this case the operator would hear sounds coming from the kitchen.
As an aside, I find the proposition somewhat preposterous that Sheila could have in any way challenged her father that morning and got the better of him. IMO, Nevill would have shut her down instantly if she was perceived as a potential threat to them all.
Clearly whoever murdered them all had to have strength and a physical presence capable of carrying out everything we now know happened.