After watching Thur's epi it is obvious that Karan is definitely Jatin. My guess is that Jatin lost his memory during the operation four years ago. In the same hospital was Karan who looked just like Jatin. He died but Jatin survived the operation. Karan's dad had the doctors switch the bodies and dadaji unknowingly took Karan's body back home. This is what makes sense at this point.
Also when Karan/Jatin was in the hospital in Thursday's episode he told his "Dad" that he feels some connection with the Wadhwas. I think he will continue to pretend he is Jatin for Veena's sake and move in the Wadhwa house temporarily. Considering Veena's mental situation she will keep thinking Kumkum is still Jatin's wife and expect them to be together making life for Kumkum difficult. 😡 And Jatin will eventually get his memory back in the process.
What do you guys think?