How could Sheila have overpowered and beaten her father?
She didn’t need to. Nevill Bamber was shot 8 times. He was shot 4 times in the bedroom, once on the stairs and another 3 times in the kitchen. He could have tried to get to the killer who was backing away from him, until he became incapacitated on the kitchen floor and was beaten around the head to make sure he was dead. A trial of strength doesn’t need to be involved.
Jeremy repeatedly told people around him that he hated his family and wished they were dead.
Sheila believed that she had powers given to her by the devil. She called her twins the "devil's children," and said she believed she was capable of murdering them or of getting them to kill others. She also spoke to her psychiatrist about suicide (although he did not deem her an immediate risk.)
How did Sheila kill several people with a shotgun she wasn't experienced with.
It wasn’t a shotgun. She had experience of using the gun.
How did Sheila not get any blood or broken glass on her?
Some people do clean themselves before committing suicide. Sheila had been raised very strictly Christian, and much of her psychosis and paranoia revolved around religious ideas. Cleaning oneself to meet god is not unheard of.
Sheila’s blood was found in the silencer.
Not exactly. It was found to be the same blood group as Sheila’s, but it could also have been a combination of Nevill & June’s blood, who were also shot with the gun. Presumably that means it could match one of, or a combination of, biological family memebers, such as the cousins who found the silencer.
I’m completely torn on this case, but some of the points made on this thread do have plausible counters.