So the plan is clear and the stage is set. Sona will be made to see the suffering that Ishwari has borne for her son and will forgive her for all the stuff Ishwari has done to her: told her to get out, told lies about her to Dev, and never accepted her as a member of the family, forced her to try for a baby...yada, yada.
In all likelihood Ishwari has killed someone to save Dev and Khatri knows about it. Or it could be that Dev has killed someone in rage while trying to save his mother, Ishwari knows this but Dev has forgotten the trauma. Ishwari is possessive about Dev for this reason. She wants to protect her betaji at all costs.
Ish will be shown as Mother India. I guess if they want to redeem her character they will have to do something like this. But what Ish did in the past to protect herself and her family is a natural thing for a mother to do. We always knew she was a good mother, in cloying way.
But does that make her a good person? A good mother-in-law? No it does not. And while Sona can be appreciative of Ish's sacrifice and help to mend Dev and Ish relationship (though I don't think it needs much mending at this stage of the serial... it needed that around episode 240 or so. Now Dev is back to being where is my Mumma?), I see no reason why it should make Sona forgive Ish for the role she played in breaking her relationship with Dev. Those are two separate things.
