Good questions. No, racial categories are never perfect, but as John Baker of Oxford pointed out, the existences of gradations between blue and green do not mean that "green" and "blue" do not exist.
I would say that white nationalism is about 1) self-identification followed by 2) objective measures, including the judgment of those already self-identified. I honestly wouldn't run around asking if white-looking Jews were white. But if they piped up and said we ought to fight wars for Israel, I'd be suspicious. And if O.J. announced that he was white, well, I'd have to politely ask him to leave.
The Earth is still a huge place. There's room for us all. Why in God's name must we all be crammed into the same space? WHAT IS THE MORAL IMPERATIVE BEHIND THE IDEA THAT BLACKS, WHITES, YELLOWS AND BROWNS MUST ALL BE STUFFED INTO THE SAME PHONE BOOTH? Seriously? Can anyone answer this question?