IIRC, “Sheets Byrd” actually used the N-word on the Senate floor or in some media interview once.
Byrd said “white nigger” in an interview. He was trying to make some kind of cultural point about how people talked in the South, back in the day. He wasn’t saying that HE said those words, but I believe he referenced a mother or grandmother.
Which was all beside the point, since the rap on sheets Byrd was his Klan activities and known racist attitudes. All of which were overlooked because he was a Dem power broker and otherwise a LIB.
As for the words “white nigger”, it all depends on who says them. If a Byrd says them, they could be forgiven because when he repeated them on tv he wasn’t specifically aiming them at Black people, he was talking about a cultural expression he had heard. Any excuse will do, for a Byrd. But if Paula Deen were to say them, well, her life is over. No excuses.
From what I know, “white nigger” is a derogatory term for a white person who has so associated themselves with Blacks, they might as well be one.
Another usage might be as a synonym for “white trash”, which is a derogatory way of linking a white person to Blacks - or at least to what was condemed as a lifestyle of poor Blacks.