He was sleeping 'like a log'.
Obviously if he had his bedroom door shut, doubt he would wake from the call.
Otherwise, a long wait for Nevill while Sheila was upstairs with a rifle.
No need for speculation when the soc evidence at WHF by way of blood stains/lack of, casings, distance of shots, trajectories and wound tracks tells its story by way of facts.
It can be evidenced the perp was in the main bedroom shooting June when NB appeared on the landing at the entrance to the main bedroom and sustained his facial shots. This supports NB coming upstairs from the kitchen having called JB.