I agree with you completely. Right from beginning, Ahaan has been shown as a mango principles, who follows all the norms n rules, who is an ideal son for his family so such kind of scheming is certainly not his cup of tea.
What I like about him is that he has never crossed his limits, he never plotted against anyone at his or her back. Even when he made Ishqi's life miserable he had the heart to accept it on her face. Thus, it's understandable that now though he knows about Rhea, he is still not able to believe that a girl can stoop so much as Rhea herself says that KYA MAIN ITNI GHATIYA HU..?
Unfortunately she is GHATIYA which Ahaan is not able to accept.
Talking about his being a successful businessman, this thing has nothing to do with that and then even businessmen have some ethics. In fact they do have their own principles and morals of running the business...
And remember guys, Ahaan is the one who blindly trusts his people...you are forgetting Mayank whom he trusted soooo much as they were best friends... Same goes with Rhea. He was with her for 2 years so he feels that she cannot cheat with him sooo badly.
Moreover, we have to give him some peace as off lately he has been the worst sufferer thanks to all those unfortunate events that kept on happening. So let's be with him