The physical evidence at soc by way of blood stains and lack of, casings, distance of shots, trajectories and wound tracks all support the theory the perp was in the main bedroom shooting June, whose head was on the pillow when she was first shot, before NB arrived on the landing stairs where he sustained his facial gsw's. He turned to retreat downstairs with the perp following. When on the main staircase NB sustained his shoulder and elbow/chest gsw's. It is not possible to know exactly what went before. A possible scenario is that SC locked herself in the downstairs bathroom with the rifle. NB attempted to reason with her through the door to no avail and decides to call JB to assist in either knocking the door down or one of them entering via the outside window. NB may have said nothing about calling anyone or may have said he was going to call JB/police/a.n other. You can see from the layout of WHF how close the tel is to the downstairs bathroom. SC may have listened at the door and shortly after hearing NB move away she left the bathroom and run upstairs shooting June. Meanwhile NB manages to get out "Sheilas gone crazy shes got the gun" and hearing gunshots he simply dropped the handset on the kitchen top. No matter how unlikely anyone thinks this sceanrio is the physical evidence at soc supports something along these lines. It does not support the perp entering the main bedroom and opening fire on June and NB inside.
You are relying on a lot of specific things to happen. In a vision.
Sheila locking herself in a bathroom.
Nevill attempting to talk to her through the door.
Nevill deciding to ring Bamber. To break the bathroom door down. Or get Bamber to go through a window into a room where a woman was holding a rifle.
Sheila running upstairs when Nevill goes silent.
Nevill hearing upstairs shots at the exact same time Bamber answers the phone.
Nevill spending 2-4 seconds saying 8/11 words, before putting the phone down & then taking it off the hook.