Playing devil's advocate, he is damaged goods, raised by this crazy, bent on revenge mom while falling for the very girl his mom hates. His wedding rituals by himself were that of a man obsessed. Thinking back, he really did nothing to harm Twinkle until Kunj came upon the scene as a threat.
As for Twinkle's behavior, there is a lot to be said. She strung Yuvi along telling him Kunj and her had a plan and were not going to be married. She changed her mind midstream and the farmhouse incident clinched the marriage deal for her though it doesn't make sense. Why marry a guy who tired to force you? Unless you have started falling for him unbeknownst to yourself?
Yuvi is a besotted, jilted boyfriend who turned demented at the wedding. Had they not been invited to the wedding events, would he have plotted to this extent? Now that he has gone back with his mom, is there a need for Twinkle to go to Yuvi berating him for kidnapping Ishan? Yuvi looked bewildered in the pre-cap, truly hope he had nothing to do with the lifting of the child from the yard.
Twinkle's behavior tells a lot too - she is not afraid of Yuvi, is this because she knows him well enough to realize he will never cause her physical harm? Does she know he is so crazy about her that really while he plots and frets to destroy her relationship with her in- laws, he is only a pawn being used by his selfish and equally crazed mother? There is a bond between Twinkle and Yuvi forged through the years, of friendship perhaps, and first love on his part and perhaps an infatuation from her side. She knows him well and if she can go to him time and time again with no fear, demanding he listen to her, there must be something about Yuvi that more than meets our eye.