Racist dialogue comes in a variety of forms. Does it have to match Kramer's to be offensive? Slurs against CAucasians in general are as obnoxious as slurs against blacks in general. To me it doesn't matter if Chris Rock and his ilk never singled out a white person. Their overall tone and message about "Caucis" has the same effect to me.
That depends on how easily offended you are. I've been one of three white people in a comedy club with a black comic and an otherwise black audience (the other two were also at my table), and we got a little grief, but it struck me as good-natured. We weren't called names or insulted.
If you draw 0-100 scale for "offensive," Richards' rant was a hell of a lot closer to the top end than anything I've heard from black comics. There is a valid point that white comics can get away with about a 20 and black comics can get away with about a 40, but that's no defense for Richards, who hit at least an 80.