Aren't sniffer dogs reliable,then ? There was a dog-handler outside WHF when Jeremy was there,,but the animal didn't flinch. These dogs are trained to sniff out the most minute of residue left behind,,as you know,,their noses are far more powerful than ours.
It would also have detected stale blood,excess body odour which is given off in such circumstances as having exerted yourself beyond the norm, which is likened to a nervous sweating.
I can partially agree that Sheila hadn't shot herself,,simply because she was shot by someone else,,but I reiterate,,,it wasn't Jeremy.