I will try to include all the points U asked..
How Bhoomi changed?
Well Neha decided to quit Saarthi n so the directors thot of plastic surgery...In the show, Shefali plans on killing Bhoomi to get her out of the way so she can cum closer to Arjun..Shish (guy working under Shefali) throws Bhoomi frm a bridge but luckily she survives...Devika Parehks car seems to be stuck n thats where Bhoomi was thrown...Bhoomi manages to ask for help n Devika rescues her...Paying little attention to the roads, Devikas car meets with an accident n both the victims r severly burned...Devika dies, n Bhoomi survives...Since her face was severly damaged by the burns, plastic surgery was necessary but since no one knew she was Bhoomi n mistook her for Devika cus she was holding a photo of Devika n her kid in her hand they replaced her face with Devikas...n now the stry is moving fwd....
Why Arjun is gud with Shefali?
Well she agreed to carry his n Bhoomis child since Bhoomi had difficulties n so he thinks shes all a nice person n also cus Shefali faked all infront of him that she left Yuv n evrything jus to give birth to the child...So I guess u can say he feels indebted to her "kindness"
Hope this helps...