Affirmative action is racial discrimination

Nothing I said either explicitly denies nor implicitly contradicts that. If you think otherwise, I've failed to communicate; could you let me know what gives you that impression, so that I can clarify?

I'll make a first attempt at clarifying right now, just in case it helps. Suppose you're arguing that Saudi Arabia should improve its religious tolerance, and someone points to an obscure case where someone in Saudi Arabia somehow managed to discriminate against Muslims and says "yeah, let's improve religious tolerance; we'll start by fighting discrimination against Muslims". Discrimination against Muslims is religious intolerance, but making it a priority in Saudi Arabia would be nuts because most religious intolerance in Saudi Arabia is of a very different sort.

I am suggesting that someone might reasonably think that "yeah, let's reduce racial discrimination in the US; we'll start by getting rid of affirmative action" is a bit like "yeah, let's reduce religious intolerance in Saudi Arabia; we'll start by getting rid of discrimination against Muslims".

(Would they be right? I don't know. Perhaps they underestimate the scope of affirmative action or overestimate the amount and impact of other racial discrimination in the US. But I don't think they'd be crazy.)

I don't quite understand you here.

Your argument (in so far as you made one) appears to rely on the idea that if someone holds that "race" as generally understood is a biological unreality, then they should think there should be no discrimination on the basis of "race" as generally understood. I think that idea is incorrect; someone might hold the first of those positions but not the second, because discrimination on the basis of "race" as generally understood doesn't need to be based on (real or imagined) biological differences between "races". I gave some examples of kinds of discrimination with other bases.

Your possibilities, by the way, are all testable.

Good.

[t]he way to stop discrimination on the basis of race is to stop discriminating on the basis of race

First: thinking that "the idea of race" is all wrong is not the same thing as wanting to stop discrimination on the basis of race.

(In two ways. 1: one of those things is an opinion about matters of fact, or possibly definition; the second is a preference about what happens; the two obviously can't be the same. 2: someone opposed to racial discrimination may none the less prefer a combination of two opposed discriminations that kinda-sorta cancels out a bit, to just one of the two, even if their ideal would be to have neither.)

Second: although "the way to stop X is to stop X" sounds obviously right, if it's meant as more than a tautology -- if it means "the most effective way to make X go away is always to find instances of X that we are perpetrating and stop them" -- then I think it's incorrect. Suppose most X, or the worst X, is being done by other people; then your most effective way of addressing it may be to go after those other people.