Nevill was most likely shot four times in kitchen. The trajectories to his head are all similar. It's far more likely they never found the fourth casing. The alternative is Nevill moving around with a head shot, very unlikely imo.
But if there were 13 cases in the bedroom, and one just outside the bedroom, making 14 in total, 7 would be associated with June, 2 would be associated with Sheila, and the other 5 must be associated with Nevill, so how can he have been shot four times in the kitchen?
Malcolm Fletcher also recorded a total of 13 bullet cases in the bedroom, and one on the stairs. He didn't specify which stairs, but DC Hammersley said it was on the landing outside the bedroom.