I think Arnav will get very much disturbed but he will say something to the effect " This is not Khushi's fault. I'll marry her." When he will start for the mandap ( remember it is not in RM , but somewhere where you have to go by car) Shyam will play his dirty trick ( already has experience in fiddling with brakes of the car) and because of some dirty trick like this , Arnav will not reach in time . Shyam will go to the mandap and tell about Garima's role. Khushi will assume the worst and for her sake ( and Arnav's sake too --- she may think it is better if she goes away ) she may walk out.