The safe word violation really sour the story.
I understand it adds weight into the pressure the two adults try to push Nana and Kaory forward, and it's kinda in character for them as people who has trouble conveying what they want to say; but, violating safe word is just really not good.
Author could have played it that safe word never been established so Nana's beg to stop is never clear so Kaoru has to interfere, but the weight is far less than safe word violation and might not seems enough to push Kaoru to go up against his idol like Sarashina, but... yeah, safe word violation just sour the whole thing.