The last two are not possible.
Had June been alive for a period of time after she fell on the floor. We would expect to see a lot more blood in that area. Once you are dead your blood pressure and circulation stops and it wont spurt out as much.
The bloodstains on Sheila show she was sitting up while both shots were fired. Hence it must have happened quickly. If she was shot once and then a killer reloads she would in the mean time have covered her neck with her hands and coughed up blood, creating a mess and smearing blood everywhere.
Alive or dead, June was on the floor for several minutes before Bamber's 6th & 7th shots into her.
Bamber was preoccupied with Nevill in the kitchen. Having the fight, re loading & shooting him again, putting him onto the coal scuttle & burning his back.
He may have automatically shot June twice more after entering the bedroom again & seeing she had moved from the bed, assuming she was still alive. With Nevill gone & June shot 5 times, he had time to make June's 6th & 7th shots much more lethal.
Sheila was knocked out but still breathing after the first shot. Bamber had no idea how long she would stay alive or when the police would force entry. So with no resistance from Sheila, made a more successful second shot.