I was thinking..its just a thought so please do reply with ur views...ok..back with what I was thinking đ The mystery lady is Karan-Nandani's daugher..I know some other ppl have also made posts that it might be her..My reason being..Remember before the Leap..Mandira told Meera and Nandini herself that all she wanted is that baby which was in Nandini's womb..I think she probably kidnapped or took Nandani's baby..she was raised by Mandira, that's why she is againt Virani Pariwar and she is going to follow the footsteps of her Dadi(Mandira) and ruin Tulsi and that family..She probably fed all this B.S in her brain that she is so aggrevated and raging to take revenge..
OR
The mystery women is not Mandira herself however she hired someone else to harass them...
This is my view..