I'll get to the meat of your question, but two sidebars first:

> My morality follows from basically “discrimination based on immutable personal characteristics is always bad”

As a bit of an aside, are you saying that this is your base moral precept from which all other decisions about what is or is not ethical follow? This would be a very strange ethical framework (for example, it follows that murder is ethical). Usually moral and ethical frameworks follow from very high level principles "optimize happiness/utility" (utilitarianism), "do good things" (deontology), etc. It seems like you might be coming from a rights based ethical framework, but it's not clear.

> How would you convince a White Nationalist that such discrimination is bad while leading by example that other kinds of discrimination are just fine?

I don't generally waste my time arguing with people who are engaging in bad faith[1]. In this day and age, I don't generally believe that a white nationalist is going to be convinced by facts and logic, or really anything else. I'm more concerned about appealing to the majority of people who aren't white nationalists, because ultimately I believe that discriminating against white nationalists is okay[2].

Ok, now back to your actual question: in what cases is it okay to discriminate? I'm approximately utilitarian, so the simple answer is "when the expected value across society is positive." For me, I don't think optimizing for strict utility is the right goal, instead I personally think that we should maximize agency (possibly median, possibly total, unclear exactly), which is something like each individual's ability to make meaningful decisions.

This sidesteps issues of de jure rights. It leads to conclusions that we should do things like lift up the least fortunate and provide social safety nets, since a destitute person has less agency than a person capable of getting by. Similarly, it addresses questions of when regulations on personal liberty are ok. If allowing some action has a reasonable possibility of reducing agency of some other group, we should give it less protection than another action which does not.

So for example, when you have two groups, racists and some racial minority, it makes sense to provide the racial minority more protection than the racists, since the racists want to reduce the racial minority's rights more than the racial minority wants to reduce the racists rights. That is, the minority would prefer it if the racists weren't allowed to be racist. The racists would prefer it if the racial minority weren't allowed to live in the same area, or use the same water fountains, or what ever.

That's an extreme, but illustrative example. So back to the less extreme position: Discrimination is okay when it's done explicitly to counteract a gap between what the law (or more broadly society) intends and where society actually sits.

Society intends for hiring to not discriminate based on race. But it did for a really long time. The put (and continues to put) the subjects of discrimination at a disadvantage. If you're deontological, then "nondiscrimination" is the rule and that's that, but if your goal is justice or utility, then discrimination to bridge the gap between intent and reality is ethical. It should be done carefully of course, but it's alright.

If you want to see a fun set of examples of this, voting district and gerrymandering law in the US is full of weird examples of trade offs between the need (under the Voting Rights Act) to allow racial minorities to elect representatives of their interests, with the current opinion of the USSC that any districts can't be drawn based on racial lines (except in very specific circumstances)[3]. In other words, States must draw districts to preserve the interests and representation of minority groups, but can't draw districts based on where those minorities live. It's weird.

[1]: https://en.wikiquote.org/wiki/Jean-Paul_Sartre#Anti-Semite_a...

[2]: https://en.wikipedia.org/wiki/Paradox_of_tolerance

[3]: https://www.ncsl.org/research/redistricting/redistricting-an...