Re the silencer, my question would be this:
If Jeremy did indeed remove the silencer because he realised that Sheila couldn't shoot herself with it attached to the gun, why would he put it back in the cupboard? Of course, if he shot Sheila with it on, he might have realised that her blood might be found in it, so it would be best to hide it, but in a gun cupboard where anyone could find it? He would surely have known that blood could have been found in it quite easily.
Not only that, but even if he thought it would be a good idea to pretend the silencer hadn't been used at all, why would he put it in the cupboard with visible paint and/or blood on it?
Why not just take it with him and hide it somewhere? If anyone asked where it was, he could just say he didn't know.