He is not as good as he could be at acquitting himself, that's true. However had I been him, I would have said in regards to who was to blame for the gun going off (as opposed to Reeva being killed), "Can you qualify what you mean by blame? If you mean was I to blame from the standpoint of intention, then no. No one was to blame for that, it's just something that happened. It was just the way the sequence of events and mistakes played out. However if you mean was I to blame for accidentally killing her, then yes that is the case."
He had to be careful there because of the legal noose Nel was trying to force him to stick his head into. And I think it was more a case there that Nel was more confident the semantics than OP is because of his familiarity with legalese etc. Again, I don't it showed one thing or the other. It was just more court theatrics.
I think it may have pleased the judge more if he had something like " I take responsibility milady for Reeva being shot when the gun went off but I did not intentionally shoot her"... rather than make out that he was without blame. Is there really a difference between taking her life and killing her?