Fasts missed during one's teens

Q: I am 25 years old. I missed out fasts during my teen years and I am unsure of how many fasts I've missed. Probably around 30-40. I do however want to give kaffarah for those fasts. Should I re-estimate how many fasts I have missed and pay a kaffarah for each fast, or should I pay one kaffarah for all the missed fasts? Also, how much should one pay for a fast? Or how many people should I feed to suffice for one fast? Jazakallah in advance.

Dawat and tabligh

Q: I have a question about importance of dawat and tabligh. I want to know if tabligh is fard or wajib or anything else? Tell me anything about dawah in Islam in Shariah. I want to ask that if I don't do dawah do I go to paradise or am I a real muslim that Allah accepts?

Ahaadith relating to Dajjal

Q: I just want to know about Dajjal. Some people narrate hadeeths about him being a son of Adam and will be born to a couple who would be barren for 30 years while others narrate a story about him being chained on an island. What should we be believing? What's correct and what's not? Please do enlighten me.

Returning the mahr back to the wife after divorce

Q: I wish to ask regarding the divorce. One of my friends divorced 3 years back and hooked up with another woman. His first wife was not a good woman. While she was with my friend she was keeping an affair with another guy. My friend got to know this and he divorced her and her parents gave back all the gold and mahr money as well but he is not aware. He would like to know should he return the mahr money or not?