Dev probably would not die.. If not for a lead, they would surely want Nithya/Durga to have someone left from her old days who really loved and cared for her and her family. She currently has only Dayal and he knew her only much later in her life. So Dev may survive if only to go into a coma or something and wake up just before the end. One can never tell!
As for the story, Nithya/Durga certainly will be losing/almost losing some of her moves. It was easier in the start because she came in as a surprise and was unknown but now the Goenka's will surely lookout for any trouble. They have not survived this long and built the reputation of ruthlessness without reason. If they give in without a fight, then that would surprise me.
The cat and mouse game is on both ends. Unfortunately for Durga, there will come a time when she will face a real dilemma between her want for revenge and the cost of revenge which may include Dev, Payal and Dayal.