Sort of echoing or at least inspired by tehstone's comment, people in America have this weird masochistic enjoyment from being offended. Words really only have the power that you give them. Saying pretty much anything is bound to offend someone. It doesn't usually make any difference what you mean. It usually comes down to what the actual words are and who says them. Americans are terrible at incorporating context into their judgement.

To answer your specific questions - usually people don't call each other nigger if they're trying to be friendly. They'd go closer to nigga for a friend. See Denzel Washington in Training Day for an example.

People like to whine about retarded because they can. Now we're supposed to say "intellectual disability." Means the exact same thing, so to me it doesn't make a difference. Retarded replaced "feebleminded" or "idiotic" as a medical definition for people with developmental disorders. Now we've replaced that with intellectual disability. Calling someone retarded isn't considered on the same "level" of offensiveness as nigger, IMO. That's pretty subjective, though.

Context and your audience determines your word usage. Losing a game of Mario Kart and calling someone an asshole is different than calling someone an asshole after they key your car.

If you're offended by "you guys," fuck yourself so hard (not you, but... you know. Those folks that are.) Asking "hey, why do you guys tip" and getting that response is ridiculous. If you can't answer a simple question, then it's time to go meditate or some shit because you are way too high-strung.

For some reason, Americans as a whole don't know how to give people the benefit of the doubt. People go immediately from hear word to react, skipping any kind of thought about context, meaning, or speaker's background (e.g. foreigner).

I didn't think you were angry or frustrated at all. I understand that you're genuinely asking these questions and providing your own experiences and thoughts as context.

Edit - If you're comfortable saying, where are you from? Just curious!