BonnieL the toilet cubicle is so small there is nowhere to hide and therefore avoid the bullets, also the bullets would, and one did, ricochet on contact with the walls.
Also, it appears as if he changed aim after the first shot on hearing the person fall.
The type of bullets in the gun ensure death or at the very least critical/life threatening/incapacitating injuries on contact with a human or animal, and OP was well aware of this.
The shots went through the door at torso height, the largest mass on a human and the easiest one to it when aiming and immediately incapacitating. People who are shot in the stomach rarely survive.
If a person had crouched in the cubicle the height of the shots would have probably hit them in the head.
There was no escape from the toilet cubicle, except into OP's gun sights.
In order for the intruder, panic, shot wildly in fear, scenario to work OP has to be unable to see said intruder.
I'm not sure I understand this bit of your post "He'd have made it much easier for himself if he'd just opened the door and shot (in the scenario of knowing it was Reeva). I guess I'm just thinking that there would be many more easier ways to do it, and get away with it, if murdering Reeva was his intent. Which is part of the reason I don't think he knew it was her."