If they wanted to kill Mangala, they could have killed her in that fire. Without Nandini knowing all her lies.
Nandini would have truly mourned like a daughter. What is the necessity to fill that little girl with all those things?
Did Nandu know how his father behaved with his mother during the initial phase?
Did all the kids know how DS treated Anandi during her childhood? And how DS insulted Sumitra and Bhairon? How she insulted and tortured Bhairon's wives?
etc. etc.
Why only in Mangala's case, they made her a completely negative character in Nandini's eyes.Her death in the fire would have closed her chapter without damaging her image much. And Nandini would have been with her mother.