Whether or not wearing nun/priest costumes or a father headdress is ok, I think it’s quite different from using elements of music from other cultures. The Chiefs fan is wearing a headdress that is clearly intended to portray an Indian chief. That’s taking a cultural symbol and erasing it of meaning just so you can have a costume stereotyping another group. He’s certainly not engaging with the culture. On the other hand, using and remixing cultural elements that you interact with is part of how cultural contact works. In some parts of Africa, corn rows and dances similar to twerking are common. They seem to have quite a different meaning in the US. Similarly, Africans often take elements of American (particularly black American) culture and make them their own. Some Americans take elements from other Americans who don’t look the same as them, then remix those elements, thereby making those elements broader and healthier.
White people’s rock and roll may have been inspired by black music, but they took it in directions it wouldn’t otherwise have gone. Where recording companies decide to promote a white group singing black people’s music over a black group that’s at least as good, because they want the music without the black people, this is a problem. It’s not necessarily the fault of the white group though, particularly if they’re honest about their inspirations. If handled properly, it just means that the music has a more mainstream audience, not that black artists are denied their own audience or artistic integrity.
I think this is a good point, but in some ways these are two separate issues. Do people who are not black appreciate and remix black culture? Yes, and that’s a healthy thing. Do some people not care about black lives? Yes, and that’s unacceptable. Are some people in both groups? Quite possibly, which shows their cognitive dissonance. This has nothing to do with the legitimacy of saying that a modern music style belongs to a specific group though.
To an extent, but not all artists are historians. They encounter sources of inspiration and make them their own. This is universal and has nothing to do with privilege, although some people are forced to know more about dominant cultures and some groups are looked down on, despite their work being appreciated in isolation.
Elements like rhyming structures or melody are particularly not the domain of one group; they seem to be much more a non-infinite series of possibilities. Here’s an old white guy using black rhyming structure before it was marked out as such.