I have a strange feeling that Manthan will be the first to find out it is Mayank. I have noticed that Manthan has a certain level of respect for Damini, when he finds out that she was lying to protect her pagal son and get him into trouble, I think he will explode and wont spare either of them. You know what he already hates that Mayank, and he will definitely be more pissed at him and Damini than Tulsi, since Tulsi got duped as well.
What is going to happen is KT and Laksh marraige will definitely get cancelled. KT will refuse to Laksh. Damini will realize the strange affection Mayank has for KT, and will take the proposal of their marraige to Tulsi. KT will agree as well because she thinks Mayank is so innocent, and she can take care of him, since he is alone just like her and they can have a good life. But Manthan knows what Mayank did and is going to seriously screw up Mayank by kidnapping him on the day of marraige.
Then I am going to go out on a limb here and suggest that Manthan more out of revenge will agree to marry KT, Manthan will convince Tanya he will put an act, and Tanya will convince everyone. KT will be pissed ofcourse she wont realize what is happening. On the wedding night ofcourse Manthan confesses everything. KT is shocked then asks why did you marry me, he tells her to get revenge on you and that pagal Mayank. Mayank doesnt get what he wants, and I also wanted to punish you and make sure by marrying me you are not happy.
Of course KT is crushed, but now she is more worried about Mayank. Mayank makes a comeback from the overnight kidnapping, he is furious that all his acting just went to waste. But he continues to stalk KT. KT will try to escape him, who knows Manthan might help her or not, but knowing him he will probably be enjoying her torment.