if the intruder story is true, there would still be an element of doubt surely (or if not doubt, then hope) until he broke the door down to see.
Yes, but I think this is why Nel is digging her. How much of an element of doubt did he have? Does his behaviour reflect someone who thought she might be in the toilet toilet but strongly doubted it, or does it reflect someone who was panicking that she was probably there, but really hoped not.
It's a subtle distinction, but an important one.
If he thought she was probably in the toilet, when did he start thinking that and, critically, why?
What happened between him discharging his weapon, and his sudden belief that Reeva was behind that door?
In his story, he started thinking it was probably her when he realised she wasn't in the bed. When she wasn't on the floor, or behind the curtain, it confirmed it. But this is at odds with the version where he continued to hold his weapon and keep the lights off.
This is why Nel wants to pin that moment down (maybe, I don't actually know). What happened between him discharging his weapon, that made him think she was in the toilet simply because she wasn't in the bed?
It's possible that what he's digging at is this; Pistorios didn't have his gun with him because he left it in the bathroom, and he knew she wasn't in the bed, and didn't need to do a thorough check, because he knew she couldn't have been anywhere else but in the toilet. If this is the case, then what happened so that he knew that?
The most likely scenario is that she screamed. If she screamed, it could only have happened before the shooting started, or after the first shot. If that is the case, and he still fired three more times, it's premeditated murder.
I still don't know what's true or not, but the moment when Pistorios knew she was in the toilet is key. If it was, as he said, not until he got the door open, then the darkness, the gun being carried around with him, him checking all over for her, all of it makes more sense.