They are importantly different threads. The shooters being different.
There is no guarantee either Sheila or Jeremy started shooting in the main bedroom first. David suggesting Sheila started shooting the twins first. Neville running downstairs to ring Jeremy.
However it is almost certain that Jeremy would use common sense and shoot in the main bedroom first.
Evidence establishes 10 shots were fired in the master bedroom at June and Nevill before the gun was empty and after it was empty Nevill and the killer went downstairs to the kitchen. There is no way that the gun could have had 10 bullets in it unless they were the first victims. Nor would June have been in bed had the parents heard prior shooting.
This evidence is note dependent upon who the killer was. It makes no difference who the killer was this is what happened regardless of the identity of the killer.
Having 2 threads that ask who was shot first are redundant because who was shot first is the same regardless. The ONLY difference in the order of killings dependent upon who the killer was is the final victims. June died first then Nevill no matter who the killer was. If Sheila was the killer then the boys would have to die next and her last whereas Jeremy could either kill Sheila 3rd and the boys last or the boys 3rd and 4th and then Sheila last.