Relatives find the silencer. Realising that Sheila could not or would not have used it, they plant blood inside inside it to incriminate JB.
Relatives report silencer to the police.
Police give silencer to lab.
Lab look at silencer un-aware of the circumstances it came it into being and conclude Sheila was murdered.
Done. Nothing industrial about it.
The industrial frame would involve the relatives, police, Julie, & experts. The police would have had to create a vast industrial frame department, weeks after the massacre.
The only advantage the police would have is that Bamber was the only alive suspect due to Nevill's 'mysterious' call. Bamber having motives, opportunity and no alibi.