Why do you think that she didn't want Khushi for Arnav? I think she absolutely did. In her eyes not only was Khushi the epitome of a perfect Indian girl, polite, sanskari, respectful of elders, but she was spunky enough to give it back to Arnav as good as she got. I think that first confrontation in RM, when Khushi turned in the resignation letter was the eureka moment for Nani. That is when she decided that Khushi would be perfect for the family and Arnav. Plus,the cherry on the cake was that she was from Lucknow!!! What more could Nani want?
Okay answering this one thing before I go. Visiji, if she wanted Khushi for Arnav, never did she ask Arnav to reconsider his choice. She was happy with Arnav declaring that he is going to get engaged to Lavanya in Diwali function. For her Lavanya was acceptable once she started respecting elders and she defended Nani and her beliefs to her friends. And Nani was equally happy on hearing Khushi's engagement. So I doubt she did want only Khushi for Arnav. If so, she would have tried to persuade Arnav to think again once on his decision or would at the least felt bad on hearing Khushi engaged. Once both of their engagements were off, and the family saw the teasing and simmering attraction between these two at the havan time, that's when Nani firmly wanted Khushi for Arnav. Before that, I doubt she was this sure. My feelings.