Is Stephen Miller A White Nationalist?

Not likely, but possible.
   The SPLC is, of course, a perfectly unreliable source. And AOC's opinion counts for almost exactly nothing. But even still, it's a reasonable question.
   Basically nothing that's blared from the MSM yet proves the case--nor does it come close to doing so. The Pope has, basically, called for open borders. And that would be a catastrophe for Europe and the U.S. I haven't read Camp of the Saints, but it's actually pretty highly-regarded just as dystopian sci-fi. I ought to (and might) read it, just to see what's really up with it. Referring to it while making the case against open borders is, obviously, not inherently bad nor racist. And Miller makes a perfectly reasonable point about the Soviet flag. As for the point about TPS, I suppose I don't see what the fuss is about that.
   Remember, progressives think that anyone who is in any way against any form of immigration, legal or illegal is racist. This is well-established. Also beyond doubt is the fact that progressives will spin anything as racist that can possibly be spun that way--unless it's said by someone they have an interest in defending.
   And: every one of their accusations against Trump that he said something racist have turned out to be bogus.
   But, again: none of that shows that Miller isn't a white nationalist.
   However, none of the evidence proffered shows that he is, and that's what's decisive. It doesn't even come close. Everything he's said is perfectly consistent with him basically being Trumpian--roughly, a civic or constitutional nationalist who believes that open borders would be disastrous, and that many aspects of the current system are being abused.
   And, of course: the way the progressive game is played is: if they want to get you, they go through everything you've ever written, including outliers, including in anger, and they find anything that can be spun as racist (etc.). Of course, there will always be something for anyone. Then they declare that even the most outlandish interpretations are basically incontrovertible evidence.
   We absolutely, positively, can't have actual white nationalists in power. There's no disagreement about that. But the left's strategy on this is beyond any doubt: anyone who wants any sort of immigration controls of any kind--by Western countries, anyway--will be portrayed as a virulent racist.
   Honestly, given how utterly disastrous that view and the attendant rhetorical chicanery is, I think we should be a lot more worried about that than whether Stephen Miller has unimpeachable attitudes. Immigration insanity is real, it's being pushed hard by progressives, it would lead to catastrophe, and like the rest of progressivism, it comes along with a kind of self-sealing defense tactic: no one is permitted to disagree with it nor criticize it. Anyone who does is a racist.
   If Miller's a racist, get rid of him. But also absolutely, positively oppose progressivism with all your might as well. Miller's the smallest of potatoes compared to the other guys.