In SBS they show Arnav entering the Gupta house. Elsewhere they show Khushi slapping Shyam. If you put two and two together, there is hance that Arnav sees this tomorrow. The whole slap scene is not in RM, but in Gupta house as shown in SBS. So this is possible
I wish :P .. but I don't think so because it was just offscreen masti to me and other people. If you take a look again, she laughed after and so did he. It was just fun stuff! Just like her and Barun when she hugged him.