Before answering all your question, let me ask you one of mine.
Why do we think Dev loves Sonakshi or even gives a hoot for her?In my opinion, she was his obsession. Now he has a new one. But still he doesn't share even his old toys. So he wants the new toy, but also doesn't want to let any other kid play with the old toy.That I think answers your why Sonakshi should not bother.Now about Ishwari, getting another kid out of a woman requires her to allow another woman. Until there was no kid, she was willing to let some womb walk in. Now there is a kid minus the unnecessary accessory. So why bother? Just grab the kid.That's Ishwari's why.Now about Dev, I have no answer. Because he is in my opinion is not even have any compassion for anyone except his nikamma family. If you ask me, he deserves them. Needless to say, Soha didn't get her compassion from him.