I think Sona was right to make Dev sign the contract. He jumped into the window and tried to take his daughter, any mom would freak out when her ex tries to do that. She was only being protective. She did let Dev take Suha and spend time not restricting that but slowly and not to disrupt Suha.
Regarding the clause about not knowing his daughter and Dev responsible for it, I think she put it there so that tomorrow if there is a custody battle there are no allegations that Sona hid the Child. Yes Sona could have tried more.. but even she has her self respect.
After Suha is grown up of course she if afraid what nonsense Iswari will do , especially after she gave those statements last time like I will do anything for my granddaughter. Ishwari is manipulative and Dev falls for that...As a mother I think what Sona did was right.