Yes, I thought the Nihilus attacking Sion (or vice versa) wasn't necessary. I am on board with your focus. :}
I don't think Sion needs to respond to her either. If anything, Visas should probably mutter a "Yes, Master" type of line to Nihilus after he "calls her off". (I could try to find an appropriate one for you.)
I am not sure how you see it but what I took from away from the scene was that Visas Marr is mostly a witness to this conversation and not really a full participant.