The first two points pretty much hit the nail on the head as far as I can see. Your third point is interesting, although I don't think it's something people generally think about, so I don't think it's a factor when we're trying to figure out why it is that we're so squeamish about race. People love putting other people into little boxes, essentially condensing them down to a few characteristics. It makes them easier to sort through and think about, as well as giving us other people to compare them to.

There was a woman speaking at my university for Women's Week. She was talking about racism and struggles of black people in modern day Ireland, but when she was about to say "black people" she stopped, choked up, and said "persons of colour" instead. I think to most people, describing anybody as "black" or "brown" isn't a problem. In fact, as a brown guy "person of colour" sounds a little bit silly. I think it's purely people's fear of being racist. Reginald D Hunter did a pretty funny piece about this phenomenon.

I don't know about anyone else, but from my own experience I found that white people become offended more easily than others about race jokes even when those jokes aren't about white people. For example, I grew up abroad in a very international school (over 77 different nationalities if I remember correctly). People were constantly taking the piss out of each other for their race.

When I moved back here to Ireland though, I remember jokingly calling myself a walking Asian stereotype (really good at math, plays piano, lightweight drinker, can't eat anything without rice, etc.) and the (white Irish) person I was talking to saying "That isn't funny, that's racist" and walking away.

In general though, I have found that type of person to be a very, very vocal minority, which I think gives the impression that it's the general narrative at the moment. I for one am not planning on losing any sleep about it.