Gheebat and Mocking

Q: Is it kufr to backbite a Muslim to a non Muslim? Also is it kufr to complaint against a Muslim to a non Muslim if he has harmed you for the purpose of correcting him so that he doesn’t do the same mistake again?

Q:

1. If someone is short or black, will laughing at them be kufr because Allah made them like that? It will be as if a person is indirectly laughing at Allah's decision.

2. I heard in a hadith that Moosa (alaihis salaam) was dark in complexion (black or something). Will it mean that making fun of someone who's black because he is black like indirectly laughing at Moosa (alaihis salaam) too.

Q: I spoke rude to a Muslim brother and he is my friend also and he felt insulted. After I spoke to him and said "please accord with me if I said anything wrong" and lastly I said salam... but he replied nothing. He did not even reply to my salam. Am I a culprit in front of Allah? If yes, what I have to do the next I see him?

Q: If a father says to his son "if it's true that you committed what I suspect you, May Allah curse you" and surely the son committed what his father suspected him (Though the father has confidence his son will never do that due to his (son's) good character). Now:

1. What is the ruling on that curse?

2. Is the son still a Muslim?

3. Can the son, despite being cursed pave his way to paradise?

4. If surely Allah cursed a person, he distanced him from his mercy, how could that son draw nearer to Allah's mercy.

5. The father is quite happy with the son as I write this, because he obeys whatever he asks him to do, not knowing the son is cursed by him. What is your advice to the son?