Gheebat and Mocking

Q: If I insulted someone in my mind or heart without speaking it out, am I going to get sin?

Q: How can one repent for backbiting to avoid its punishment in this life and the hereafter? Does one have to inform the person whose gheebat was made and ask him for forgiveness, or is seeking forgiveness from Allah Ta'ala sufficient? 

Q: Person A tells Person B the following joke. He says the memon was in Jahannam when he met his friend. His friend said to him that you were so pious, how did you end up here? The memon replied that I have a place in Jannah, but I put my house in Jannah on rent. Person B laughed at the joke. I am aware that in Surah Tauba that there is a verse which speaks about joking about Allah and his signs and this leading one to come out of the fold of Islaam. In the scenario presented above, do both A and B lose their Imaan and in the process invalidate their Nikah?

Q: What is the punishment if someone insults the Prophet? Will he or she still be hanged to death even if he or she repents sincerely?

Q: I would like to ask what would someone have to do in a situation where he does not remember all the people he has back bited in the past (nor whom) and what to do to gain salvation in this regard.

Q: If I made gheebat of a person and I repent to Allah. The person is not aware of me making gheebat of them, do I still need to ask them for maaf?

Q: Is it permissible to eavesdrop on a conversation if I am the one being spoken about?

Q: If someone insults the Prophet Muhammad (Sallallahu Alayhi Wasallam) then should there be any punishment for him, looking at the fact that he did not kill any innocent human being, and as Allah Ta'ala says that we should forgive anyone who says bad words to us, and we should reply with Salaam? I need references from Quran and the Saheeh Hadith, only then I will believe.

Q: If somebody uses "ma shaa Allaah" sarcastically in response to a sarcastic sentence, this is surely wrong, is it not? And is this kufr? Secondly, if someone laughs at the use of sarcasm here have they done kufr?

Q: If a person has committed backbiting against someone, and wants to make taubah from backbiting, and the person he was backbiting about does not know about it, does one have to go and tell the person I said this and that about you, make maaf, or can one beg forgiveness from Allah and do some good action and send the sawaab to the person?