The simplest reasons that the gun shot came last is because of the ME and forensics. The ME testified that once he was shot he would either have died or been rendered incopacitated. We know he wasn't incopacitated during the attack because of the fact that he left the shower and got himself almost all the way down the hall and he also had defensive cuts on his hands and I believe one or more on an arm. We also know that he could not have been shot first because the shell casing fell into a pool of blood and there was no other blood dripped or sprayed on top of it. He HAD to have been stabbed first for these reasons.
For reference: medical examiner's testimony (starts about about a minute and a half into the video)...
Jodi Arias murder trial day 3. Please be warned, very graphic photos. Medical examiner on stand. - YouTube
Jodi is so adament that the shot came first otherwise she couldn't have claimed self-defense. She can't claim that he attacked her so she ran down to the kitchen, grabbed a knife, and ran BACK to the bathroom to defend herself since if she was able to get down to the kitchen (and we know he didn't follow her there) it isn't self-defense but murder since there is no reason she could not have kept running out of the house and get away from him, and there wouldn't be any reason for a knife to be in the immediate vacinity that she knew about and could immediately protect herself with. She MUST insist that the gun shot came first because any other scenerio makes it murder, not self-defense.
Personally, I think that she intended to kill him with the knife and only had the gun as back-up. I don't think she wanted to use the gun because of the fear that someone would hear it fire, and she might get caught in the house by police or a neighbor. I also believe that she imagined she could stab him once or twice and he'd die neatly in the shower with all the blood evidence being washed away. I think she freaked out after she stabbed him and he fought back, got out of the shower and was able to not only get out of the bathroom but all the way down the hall, and she went into a frenzy of stabbing because he wasn't conveniently dying right away. I also believe that the throat slitting and subsequent gun shot to the head was that she wanted to make sure he was REALLY dead, but also because she was pissed off at him for not dying neatly and quickly in the shower and made a bloody mess of the place which was not at all what she had invisioned.