I don't buy it. Only way the stomach wound makes sense is one of the scenerios I described above. Doesn't mean he isn't a bastard, but the official story is crap.
You did miss one possible scenario...
Jacobs buys a gun for himself (or has it laying around little used?), and since he stated to a reporter recently that he is fearing for his life, he wants to now carry it around with him. Amanda does not like this.
Maybe this day Dave grabs the gun as he's leaving to go somewhere and feels he needs protection. Amanda tells Dave to not bring the gun, argument ensues and she shoots him. Dave now hurt and not thinking correctly, as well as being pissed off he's shot, shoots her without thinking rationally. Realizes that he just shot his bride to be, and figures with the shit he's in with the law, the people he's about to rat on, and now his dead girl (that he reportedly was obsessed over) does himself in.
That's about the best case scenario I can think of outside of
possible cover-up.