Jeremy Bamber is not a stupid individual. If he had used that gun with a silencer. That silencer would have not even come into contention. Because he would have disposed of it. I find it very difficult at the same time to believe the police couldn't visualize from the outset of seeing shelia that she had been shot twice and not immediately become suspicious of Bamber. Thirdly, as you rightly point out how did Jeremy Bamber manage to position Shelia caffell to undertake the two shot scenario. Even if she had been incapacitated by one shot. She would have been fighting like a wildcat. Jeremy Bamber had no marks on him. It beggars belief
It's very easy to say what we think happened from a physical distance of 30+ years and a psychological distance which is incalculable. Jeremy certainly wasn't stupid, but because, it's a reasonable assumption, I'm willing to bet that, at that moment, the first shot having failed to kill her, he was completely thrown. What he may have done, had it been successful, would have been blown out of the water. It came down to plan B, but he probably hadn't planned a plan B.
Re the police and their reaction to the two shots. They probably didn't react at the time -but it may, or not, be of interest to you, that my friend's late husband interviewed Jeremy at his cottage later that day and was in no doubt about his guilt. I doubt he was the only one- because generally, we believe those 'facts' we're tutored to believe.
Why do you appear to think Jeremy positioned Sheila for "a two shot scenario". Whilst two shot suicides are possible, they're certainly not usual. As for her "fighting like a wild cat"? Perhaps you should read what effects her injected medication was having on her. I'm not certain how you come to the conclusion that Jeremy had no marks on him. Whilst clothing could protect from most, there's the possibility that there were some which weren't visible.