According to Wikipedia:The White House Murders Mr. Justice Drake said the jury could convict on Julie Mugford's testimony alone. If you're referring to the scratch marks on the mantlepiece I would agree with you that that is suspicious and would indicate some tampering of the crime scene by the relatives.
Blood on the silencer was deemed to have come from Sheila,and as Jeremy Bamber did not shoot any rabbits the previous evening unless the blood was old blood it was likely that Sheila had been shot with the silencer,Jeremy Bamber or a hitman had then unscrewed the silencer and the sight and replaced them in the gun cupboard.
None of this affects Julie Mugford's testimony.