I didn't expect that you would agree that the point had been well made and I came to this conclusion by following your reasoning so far.
If Jeremy decided to invent a call from his father he would not use a scenario that would be, as far as Jeremy would know, impossible. After messing up the suicide in such a way that he would not believe suicide to be feasible, he then according to your reasoning, invents a call which requires everyone to believe that Sheila had committed suicide. He could have invented any scenario he chose. Those options may not have been perfect but anything would have been better than messing up a suicide, seemingly making suicide impossible, but then going along with the original suicide plan despite this obvious flaw.
Your final sentence is just illogical. Are you suggesting that he didn't mess up the suicide, or is your point that he would have the foresight to know that the police wouldn't bat an eyelid at a two shot suicide and therefore his plan would "almost" work.
It is your claims that are illogical.
1) He planned to stage things as a murder suicide and once he had finished things he had no choice but to go with that option because it was too late to try to stage it as a breaking at that point. Someone who broke in would not go room to room killing everyone in their beds upon being discovered at most you shoot the person who discovers you then get the hell out of there.
2) He had no idea he botched things up, he was convinced he had carried out things perfectly. The only things he was worried about was whether his prints got on the gun when his glove came off but that was a minor issue and he mad eup the story about taking the gun out in part to deal with that possibility.
3) Though it is uncommon for those committing suicide by firearm to fire more than 1 shot it does sometimes happen and didn't lead the ME to say it wasn't suicide so the claim he should have known the two shots meant he could not still run with his suicide claim is nonsense. He had no choice and certainly was better off shooting her a second time killing her then to risk her living and telling police what he had done.
Jeremy supporters always spend their time on illogical wasted arguments because they are unable to refute the evidence in chief, it is a distracting measure. What needs to be done to establish his innocence is to refute the evidence in chief but that can't be done so it is simply dismissed/ignored and game played.