I was agreeing with you until the last part. You can't surely believe Nevill would rely on someone whom he suspected was trying to kill him, even in extremis.
If NB thought JB was trying to kill him it begs the question why he was so lax with firearms security. In any event NB never said he thought JB was trying to kill him. We are told by RWB and BW that NB made a couple of comments which have been twisted to imply he was referencing JB.
The soc evidence by way of blood stains/lack of, casings, distance of shots, trajectories and wound tracks supports NB phoning JB by virtue of the fact the shooter was in the main bedroom inflicting gunshot wounds on June when NB came upstairs. If NB was not downstairs phoning JB what was he doing?