Gheebat and Mocking

Q: How can I truly repent from backbiting? I can not find the person I have talked bad about.

Q: Is it kufr to mock the way someone smiles.

Q: Is mocking someone's appearance considered kufr because we are mocking the creation of Allah?

Q: Gheebah is a very great sin. الغيبة اشد من الزنا.

So lots of people take it lightly and will still end up speaking bad about people and ask that person forgiveness afterwards etc.

When a person speaks bad regarding another person, if it falls under gheebat, the persons good deeds goes to the person he has spoken ill about. Now if he asks that person forgiveness, even if the person forgives him, will he get his good deeds back? Or will it be such that he can now ask Allah for forgiveness as the person has forgiven him, but he'll have to restart his good deeds?

I feel that people don't realize the gravity of the sin of gheebah, and take it very lightly. Please advise. I'm not an aalima so I am afraid if I'm thinking wrongly so I dislike saying anything regarding it incase I am erroneous in this regard.

Q: If A writes a story about A's sister B, and A says that B has skill issue, etc. (the story is fake), has A backbitten B?

Q: If a youngster is sitting in a gathering of adults wherein gheebah is taking place and he cannot leave, what should he do?

Q: Sometime ago I backbit and engaged in alot of idle talk about alot of people as I was not much of a practicing Muslim. Those people are not aware of my backbiting. Now I have turned back to Allah; I have repented alot and repented sincerely and realized my mistake.

It turns out that the people I used to backbite are alot since I dont remember them all can I make dua generally for all people that I backbit without taking their names or mentioning them? like "Ya Allah, forgive all the people that I backbit and raise them in ranks".

Also, will Allah Ta'ala forgive me? I am afraid if I tell those people that I backbit them it will cause some hatred in their hearts for me. So i decided not to approach them and repented sincerely. 

Q: Is there a difference between venting to someone and making gheebah?

Q: Is it true to say that one should not spread bad news even it is in the public domain and true?

Q: I need to know Islamically whats the sin for talking bad about a dead woman that you don't even know.