No need for divorce, as Kasturi is not ..

Robbie's wife.

Since Raunak is alive and Kasturi knows tht, her marriage to Raunak is still legally valid. Raunak faking his death makes him a criminal, but doesn't annul his marriage.

So, her marriage to Robbie is not valid, and therefore even though Robbie is not aware of Raunak's truth, he actually doesn't need any divorce from Kasturi, so whether Kasturi signs the divorce papers or not, shudn't she be aware tht it doesn't make any difference as she is not legally his wife, and the only thing is tht if she signs the papers, then Sabbarwal's and wanna-be-Sabbarwal(Anita), will ask her to leave the house, but doesn't the house and business belong to Kasturi, as Robbie transfered all his properties in her name?