Please refer to the one episode which is before the holi episode. In that Ishwari eats oily food and falls sick. D v blames Sonakshi saying you have failed as a doctor. Is this your best effort. Sonakshi replies that she did not come to Dev asking for a job. Agar aapko apni is decision par afsorse hai to koi aur doctor ko dundlena.
In the current scenario. Dev might show her the papers. They might fight again. Dev might blame Sonakshi saying "you have failed as a bahu or biwi. If this was important to you then you would have tried harder. Found ways to make it work. Is this your best effort. you promised you would fight the world for me."
Sonakshi probably would reply that she did not ask Dev to love her or marry her, because but it was him who came to her. And if he feels it was wrong then he should find someone else.
This is what triggers Dev to try to find a better girlfriend than Sonakshi resulting in him turning to Casanova.
My guess is he will never cross the limit as none will even be equal to Sonakshi. The more he will try to forget Sonakshi or find someone better than her, he will fail miserably.
Meanwhile Sonakshi will leave town, become successful and come back to Delhi to buy their house back. She will start living there and Dev will come to know of his daughter. When he tries to build relationship with his daughter, he will come to know the prenup. He will then ask his mother "tunhe aisa kyon kiya". Ishwari by this time will have realized her mistakes and say yes she was the reason behind their breakup and Sonakshi was not at fault.
Just my pov and guesses. Thoughts welcome...