I'm just curious, how do you think a person could accidentally shot themselves in the chest/stomach while trying to inflect a non fatal wound? If she wanted to go for a non fatal wound she could have shot her leg or arm and claimed the guy ran off after she struggled with him for the gun. Shooting yourself anywhere center mass and hoping the police get there in time seems pretty risky. She owned a gun and had enough ammo to indicate she may be a regular practice shooter at a gun range. IMO, Rita would know you only shoot center mass to kill.
Not OP, but many people are not well-versed in how guns actually work nor what shots/injuries may or may not be fatal. That includes people who own guns and go to shooting ranges. Few people are actual experts. A lot of people get their gun 'expertise' from tv, movies, etc.
Some people actually think that a gunshot will blow you back off your feet and on to your butt. Rarely happens. Some people think that headshots are always fatal. Usually they are, but they are survivable. Some people don't realize that a shot to a limb can be fatal. One thing that comes up with police shootings of a suspect is inevitably someone will say 'why couldn't they shoot the person in the hand, leg, etc'? As if it would be easy to shoot a gun out of someone's hand, especially if they are moving. It also can't be assumed that someone who is shot in the arm would automatically drop the gun or even be disabled enough to not pull the trigger. Also shooting in the leg actually has a pretty decent chance of being a fatality because of the Femoral Artery.
If Rita shot herself and didn't mean it to be fatal, I'm not convinced that she knew enough about location, organs, etc, to realize a 'stomach shot' would very likely be fatal. Even though there is conflict about the location and I am leaning towards the chest shot ricochet through the abdomen area based on her posture in the trunk, I am fairly convinced that most people trying to stage an injury wouldn't realize that such a shot could very easily be fatal.
Again, a lot of people get their 'facts' and 'knowledge' from film and tv. In those media, very rarely is anything other than a headshot portrayed as fatal.
ETA: Sorry, spaced and didn't reply to everything I wanted to address. Anyways, I wanted to mention that if this was an attempt at a non-fatal injury (still leaning towards intentional suicide myself though), you asked why didn't she just shoot herself in the leg for example? Well, if you're trying to sell this to police, does a leg wound work? I mean, this 'killer' knocked her out, kept her captive in her car for 12 hours, then decided to shoot her; would he really shoot her in the leg? If HE wanted to eliminate her as a witness, wouldn't he shoot her somewhere more likely to be fatal? Otherwise, why would he shoot her at all? Why would he open the trunk, shoot her in the leg and then close the trunk? The police would be much more likely to think that kind of injury was a deliberately staged lie, I think.
If you're going to claim that someone kidnapped you and kept you captive for 12 hours, only to take nothing of value from you and shoot you, the shooting motive would have to be to kill you because you are a witness. That wouldn't be something a killer would shoot you in the leg for. A chest shot is much more plausible. And, obviously, if Rita wanted to stage it as attempted murder but still wanted to live through it, she's got to pick a spot that makes sense for a murderer, yet still survivable.
Now personally, you'd still have to be crazy to take a chance of shooting yourself in the abdomen and hope you'll live. But, desperate people do desperate things. So while it's pretty crazy to shoot yourself in the chest/abdomen and hope the police get there in time, some people might actually think that to be plausible. We really can't know anyone else's mind. What might seem crazy to you and I might not seem to someone else. And, if she did it with hopes of a non-fatal outcome, maybe she was okay with the fact that she might die anyways, if it gave her family the financial security they needed?