Adam I agree with most of your assumptions regarding the shootings they are feasible IMO
Whichever point in time the twins were shot, I think they were initially shot just once each.
Upstairs Downstairs Upstairs Again
Twins 2 shots twins 6 shots
June 3 shots June 4 shots
Sheila 2 shots
Neville 4 shots Reload Neville shot 4 times Reload 10 bullets
I believe that JB thought the initial shots would kill them all hence 11 bullets set-up and ready to use after the rabbit story. JB probably thought - 1 shot each to twins, 3 shots each to others 11 shots in all. But Neville was still on his feet with JB having no bullets left! In kitchen struggle takes place, Neville knocked out. JB reloads from kitchen unit, shoots Neville 4 times, JB reloads again to 10 bullets... now in panic that the other victims are still alive as Neville was after being shot 4 times!... JB goes back upstairs and shoots the twins 6 times, June 4 more times. With June only being shot 3 times the first time it's possible she would be able to move around the room. Neville must have been the last one shot upstairs the first time, as otherwise NB would have had more shots fired at him before he made it downstairs. Sheila has obviously made her way around to her father's side of the bed for protection, followed by June. NB is shot to his left side by JB who stood in the doorway. Most of the cartridge cases were found by June's side of the bed. The C cases fall to the front and on the right of a rifle.
I don't expect everyone to agree, it's just my opinion at the moment.
It's possible he wanted to commit the massacre using just 11 bullets. Aware that people would ask whether Sheila could chamber and reload.
This could mean shooting the twins first. One bullet each. Then using 4 bullets each on June and Neville, and 1 bullet on Sheila. Making a total of 11 bullets.
This would explain why June was able to walk around after being shot in bed. She was only shot four times. Bamber only shot Neville four times upstairs as he didn't want to empty the rifle, as he wanted to keep the last bullet for Sheila.
Bamber's plans to use just 11 bullets went out of the window when Neville woke and got downstairs. After the brutal fight and having to shoot Neville 4 more times, he decided to make it look like a Sheila was in an uncontrollable crazy rage. So shot the twins more times.
He then had no option but to shoot June three more times, and Sheila twice as both remained alive after getting their opening shots.
However it's just as possible Bamber set out to fire 20+ bullets from the beginning. Which matches him getting a frantic call from Neville about a 'crazy Sheila'. Telling the police on the night Sheila knew how to use all the rifles at WHF, suggesting she could chamber and reload.