It is important for the new Kkusum to be young ( i read this somewhere). And so this is what I think is going to happen...
Kusum is going to incounter Abhay a few times and get her memory back. But no one in Kkusum will know. And since no one in the Kapoor family knows that she is Kusum... she over hears Mahi's plans and she makes her own plan to befriend Kali as someone Kali's age. And she enters the Kapoor house like this and starts exposing everyone and then reveals herself...
These are just my thoughts...tell me what you think...