Having said that, I disagree with a certain point of yours : regarding fidelity. Dev and Sona have been separated for all intents and purposes for these 7 years. Even if they are not officially divorced I really do not feel it makes a difference. At that point it just makes the divorce a piece of paper. Therefore even if dev and sona have slept with anybody else, had relationships or whatever it really is not cheating. It is as sona said in the episode.
Dev is hypocritical in fighting with sona about her alleged relationship. But honestly, I don't blame him in feeling jealous. Dev is exactly that kind of guy. What I do blame him about is confronting sona about it. Because that is not his place. Feelings may not be controlled but actions surely can.
Don't even get me started on the whole sona as detective. It just pisses me off to no end. She should have told dev about it and then just stayed the hell out. But no.. typical Indian serial with super mahan heroine. Ugh.
I feel like the writers have ruined all the characters. They had a great chance to redeem and grow the characters instead they've just assasinated everybody. they could have shown a mature dev and sona.. who both understand that they screwed up( dev by leaps and bounds more of course). Instead we get two immature, bitter, stupid leads who think they're the greatest and that they've made no mistakes. We have some sensible moments of course and then again it's like those things never happened. As if both of them have some kind of short term memory loss.