Good post. Sona signed Prenuptial agreement. I don't think Dev and Sona signed divorce papers, but they have separated on fit of anger.
Their feelings for each other are still very genuine and it can be seen on Sona's face when others talk about Dev in front of Sona, and similarly Dev's feelings changes when somebody talks about Sona. Dev doesn't allow that Girl in his Room to touch him or play with his feelings as there is still spark of attraction, compassion and love between Lovebirds (Dev and Sona).
Also, Bijoy is very much worried that Sona and Soha might land up together in Dixit house. Because Bijoy knows Sona still loves Dev, and they haven't got formally divorced. He also knows that Sona might melt one day because Dev has that ability to attract Sona.
This is my take :)