So far the main arguments against the straight forward Bamber scenario are -
Nevill didn't dial 999 - With Bamber seconds behind him.
Sheila may not have been under sedation - Although she was on Haloperidol & the rifle with silencer would be impossible to hear anyway.
Bamber fought Nevill in the kitchen rather than shoot him - He had run out of bullets.
I notice you are studiously avoiding the problem of a lack of a blood trail from the main landing to the kitchen. That's rather a serious hole in the prosecution case. It could be resolved by saying that Sheila was the killer, but you would prefer not to do that.
The reason I think Nevill didn't dial 999 is because there is no blood on the kitchen phone. The issue isn't really whether he dialled 999 but whether he made it to the phone. It seems to me it would be instinctive for him to do this under attack from Jeremy, regardless of injuries; either that, or flee the house. But he would be fleeing the house bare-foot in his pyjamas.
It's a bold statement to say that the gun fire would have been impossible to hear. I'm not so sure, even if a silencer was a fitted, because it all depends on acoustics. A moderated rifle can still have a noticeable report, even with subsonic bullets. I also don't accept she was sedated and I don't accept that it has been proved the silencer was even used. It's not even certain that Haloperidol would help her sleep. One possible side-effect of it is insomnia. We know she had cannabis in her system and I wonder what contraindications this produced. I also wonder about the effect of reducing her Haloperidol dosage by half.
I note your scenario depends on Jeremy running out of bullets. Otherwise, you can't explain why Jeremy would need to struggle with Nevill.
In my view, what's needed is:
Expert pharmacological evidence.
An incident reconstruction looking at sequencing/time-and-motion and the noise impact of the same make and model of silencer (ideally the very same silencer) using the same make and model of gun (ideally the very same gun) and the same bullets.
Expert blood evidence, to explain the almost total lack of a discernible blood trail between the main landing and the kitchen. Maybe the blood soaked into his clothes and didn't drip?