I've just had an idea about who the kidnapper could be! If its not Trishna then it could be Garima (Shasha's mom)! She's been gone a long time and she has a great motive to make Parvati suffer. Because of Parvati one of her son's are in jail, and one of them was sent to an asylum (could be reason why she want's Parvati there) and the rest of her family left cos she had no money left (the Agarwals were recently fighting over money problems and the blame was directed at Parvati). Also maybe that's why Shruti was kidnapped as shasha suffered because of Parvati, so she's taking revenge by making Parvati's daughter suffer.