I actually liked the fact Rehan was shown thinking before taking any drastic steps, I believe he has realized something is wrong and that the fault lies in Zubair hence the slap in the precap.
As for Zain, do not take his silence to mean he is going to doubt his wife's character. We have seen this many times with Zain where he takes everything in and then reacts, there is nothing to indicate he is going to be pointing fingers at his wife on Monday's episode. I believe he will stand by her because he knows how much this girl loves him and he knows Zubair's story makes no sense. I really hope on Monday it is not only Rehan who slaps Zubair, I want Zain and Aaliya to take action too.
Side note, I just loved the way Zain was only concerned about Aaliya when he saw her unconscious. Zain could care less about who was carrying her, all he cared about was the fact something was wrong with her. The look of relief that came over his face when she woke up, I was in aww over that scene. It is scenes like this that should reassure fans that Zain and Aaliya love each other a lot, he is not going to be easily manipulated by Zubair especially given the fact Rehan made it clear he was not accusing Aaliya of anything which is the opposite of what Suraiya wanted.
Sorry for the long post, I had more to say than I thought!