If he was in a murderous rage though, this also does not seem likely - if he was killing her in a rage, I do not think he would be calculating how to position himself to support an intruder version that he hadn't yet fabricated.
The fact that he was against the wall and shielded by the corner, shooting at a fairly severe angle - to me that suggests more that he was afraid of being shot at or attacked and was trying to remain at a distance and behind some cover while shooting.
OP has shown how quickly he can make things up, and I think this is exactly what he was doing all the while he was being violent towards Reeva. Both things can be done at the same time by a violent abuser .. they can be extremely quick thinking, make up all sorts of lies, and be angry and violent, all at the same time.
Re. your second paragraph, I was led to believe that he was leaning against the left hand side of the bathroom wall (as you enter it) and was only just inside the bathroom .. and as such, he wasn't really using the right hand side of the wall as protection .. I don't really have a definite, specific, location for where he actually stood when he shot at the door and am only going by what I remember OP testifying and some other things I've read. Happy to be corrected on where OP actually stood when he pulled the trigger.