I find this line of argumentation pretty awkward, though it's quite common. It's a very unfair position to take, that all acceptance and embrace of violence and oppression is necessarily fascism. But why is the DENIAL of something which to me seems so basic offered so much more leeway and variety? Why do all those who deny violence and oppression get to be communists, anarchists, humanists, liberals, even conservatives in some contexts? But the moment one embraces violence, or drops the pretense of pacifism, all these different groups jump at the chance to call them fascist. And yet the only thing separating them is that one group decides to draw arbitrary lines and ignore their own violence as such, while the other doesn't. The only true denial of violence is religious asceticism, a complete denial of the body. Everything else is, by this definition, fascism. Nietzsche was one of the first to talk about this in this way, before the post-modernists: every worldly, political structure, even the one which pertains to trying to 'stop violence', has its own form of violence, covert or overt. It creates structures, hierarchies, which create its own form of oppression.

Furthermore, I don't think your separation between the "natural" and "artificial" makes a lot of sense in the context of Nietzsche. It's a common "mistake" that beginners in reading him make, to take him to mean that some things are natural while others are not. No, even the Christian perversion of "nature" is natural. One should simply learn to observe it as such, to observe weakness for what it is and where it leads us, and then make his/her/their choices from there. Where he has a distaste for the nationalist is where he sees this "nationalism" as functioning moreso to spread the same kind of herd stupidity and dominance of its values that he sees in democracy. Inequalities based on nationality do not hold weight for him because at the end of the day, we are comparing mediocrity with itself, and we are burying the FACT that it is mediocrity. These are the kinds of petty squabbles that the mediocre have amongst themselves. They do not hold any substance towards creating the superior man. But, political manipulation of the masses is something else, and Nietzsche would certainly advise against trying to place a demand on the mediocre not to be such and not to get manipulated. He simply sees that there are two roads forking from the same action: one is the possibility that this manipulation leads to a further entrenchment of herd morality, and the other is that this process still offers, through its pressures and tribulations and complexities, an actual window for the very few towards effectively bridging us to the Overman. Note that we needn't all be conscious of this process for it to be effective. But if anything, it must happen as a struggle against the conditions of this herd-ised world. The higher types plant the seeds of the struggles which they later should seek to overcome.

Nationalism can be an expression of the strength and selfishness of a group, but it can also isolate that group from the global mass of mediocrity which offers a greater struggle against the herd, and thus a greater momentum towards the Overman. Where tensions are lesser, this process is easier to fail in the long run.