The relatives would need to know -
Sheila's arm lenght.
The rifle lenght with silencer.
........
I could go through all your list, but just to concentrate on the above two, you are actually contradicting your own argument. If it's true that this needed to be known for somebody to introduce the silencer into evidence, then why did the silencer come into play at all, whether genuinely or otherwise?
In reality, they would not have needed to know the things you mention or even had an appreciation of the silencer's relevance. What happened is that they found the silencer in the gun cupboard, and they say it was 'sticky' and had blood on it, and they realised it belonged to the rifle.
If they did plant the silencer, that would mean all they needed to figure out was that the detection of any blood at all in the silencer could potentially be incriminating, regardless of whose blood it was.