Is it not possible to get backspatter from a non-contact shot with silencer attached? Isn't it also possible that after the rifle was damaged after the fight with Nevill Jeremy unscrewed the silencer and made his way back upstairs to Sheila, killing her sans silencer?
The Crowns case was that Sheila was shot with the Silencer on therefore she could not have shot herself.
The CCRC's position now is. Well just because the silencer was not attached when Sheila injuries were inflicted does not nessisarily mean it was not on at all during the murders

The scenario you have suggested does not make sense, because if Jeremy and Neville are downstairs fighting, What is Sheila doing? You would have to argue that Sheila and the twins slept through it all. Reason being the twins were shot while asleep and if Sheila woke up she would realise they are all in danger and go to check on the twins and as a result wake them up and try to get them to safety. This is incompatible with the facts of the crime.
Another factor is one of the twins had a contact shot to the head, yet no evidence of their blood or brain tissue in the moderator