All I can be sure of is that the INSIDE of the silencer was fabricated. And through a process of elimination DS Jones fabricated the OUTSIDE. When I say process of elimination I mean the only contenders were relatives and/or DS Jones. The relatives did not have the inside knowledge or forensic awareness. Therefore must have been DS Jones.
DS Jones knew where the silencer was likely to be from AP's/JB's comments re the silencer/cupboard/case. Yes I believe he slipped back in on 9th Aug and did the deed. Leaving it for the relatives to find.

What, kinda, pricked his finger, plucked an hair and scratched the Aga.
Yes mention it to Jeremy when you write to him, it could be in the next Submission, Stan the AGA man.