He attacked Kailash because that evil kameena said something about his mother. So he attacks him! Rightly so. Vividha sees him attacking her father, so she reacts.
But why the 4 slaps? she wouldn't have been so heated if it was some random loafer harassing her dad. Yes she would be angry but not so over responsive. The reason there were so many tears and slaps is because the act was from the guy she has fallen for despite trying her best not to. At the moment there's no way out, things are getting worse between both families so she keeps herself in denial. In the precap she completely contradicts herself. One she says it feels like Atharv broke her heart, betrayed her and then later she says 'pyar toh door ki baat hai'. She is in complete denial!
P.S. One things for sure, Vikram & Shivani do intense very very well!