I think that was a judgement call on Nel's part. I think he assessed their strength and demeanor, and figured they would do more harm than good. She is a legal professional and was not going to crack. And the old man was very sympathetic character, so Nel had to walk on eggshells w/him. I think it was better to get them off the stand. He did get the culpable murder evidence with their testimony however.
I disagree.
I think Nel started to get Mr Stander to indeed crack, with the mistake or accident thing.
Could have gone on from there.