Gheebat and Mocking

Q: It was maghrib time and azaan was calling and I don't know who was calling the azaan but I had some thought going through my head about this person, someone was talking about him the other day and was saying he is (addar magaj) thats in gujrati. I don't no exact meaning in English but something about brain. He does what he wants to do. Whatever is in his head, he does what ever he wishes to do. Alhumdulillah, he is a very pious person. So azaan was calling and I dont know if he was doing azaan because he does sometimes azaan now in my mind comes his name and that he is (addar magaj ) and azaan was calling, is it kufr? I didnt say it with mouth. I wont call him so I dont really like to listen to things about people but I am worried if it is kufr. Is my nikah valid? Im not sure who was giving azaan. If it was him and that comes in my mind, is my nikah valid? I repeated the kalimah as well. I didnt say it with my mouth but now Im not sure if I said it with my mouth. Im mostly sure I didnt say it with mouth. Is my nikah valid? Is it kufr?

Q: Is being sarcastic correct according to the Sunnah and Quran? I mean sometimes sarcasm can hurt someone. Please explain in this case. 

Q: If a person attends a wedding and he hears people doing gheebat and is unable to stop them then is it permissible to sit in that place?

فلو على المائدة لا ينبغي أن يقعد بل يخرج معرضا لقوله تعالى : - { فلا تقعد بعد الذكرى مع القوم الظالمين }

در المختار ( قوله لا ينبغي أن يقعد ) أي يجب عليه قال في الاختيار لأن استماع اللهو حرام والإجابة سنة والامتناع عن الحرام أولى ا هـ وكذا إذا كان على المائدة قوم يغتابون لا يقعد فالغيبة أشد من اللهو واللعب تتارخانية رد المحتار

Q: When you use to gossip and you ask Allah for forgiveness and promise never to do it again. Should you also ask the people you gossip about for forgiveness or is the one between you and Allah Ta'ala enough?

Q: If I mistakenly, unintentionally or forgetfully done something wrong to someone, something or myself will that count as a sin?

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.