Trump's racist credentials go quite a way beyond his comments on illegal Mexican immigrants. For example, there are his comments about judge who allegedly can't do his job because he is "Mexican" (though born in Indiana):
Yes, that his ties to his Mexican heritage would suggest that he would be biased specifically against Donald Trump due to Trump's position on Mexican immigration. This is not a particularly strong argument but it's hardly racist. Racist would be some unconnected national lawsuit controversy where the judge made a questionable decision and Trump said 'what do you expect? he's Mexican.'
The judge in question was a member of an ethnic/activist Hispanic lawyer's group. Imagine if the judge presiding over a case with a prominent African American plaintiff turned out to be a member of 'The Dallas Lawyers Association for White Culture' or something. I doubt that everyone who pointed this out would be considered racist.
As Paul Ryan put it, suggesting that someone can't do their job properly because of their ethnic background is pretty much the textbook definition of racism.
The goals of the California La Raza Lawyers Association are (i) fighting prejudice against Latinos in the legal system and (ii) encouraging Latinos to go into law careers. Swap out 'Latinos' for 'white people' and neither of those goals really makes sense any more, because whites and Latinos in California have different histories and face different problems. For that reason it's unclear what sort of organization the 'Dallas Lawyers Association for White Culture' would be, or what sort of goals it would have. The analogy doesn't make any sense.
Trump has, not surprisingly, been shamelessly lying about the association and trying to paint it as some sort of Mexican supremacist organization. As you might expect, given that it counts a very respectable judge amongst it's members, it is no such thing:
All of this is happening because Trump is a bigot. I think it's quite possible that he believes in the conspiracy theory that he's concocted. However, there have so far been no signs of Curiel doing anything inappropriate:
Paul Ryan's comment completely ignores the context. The job of a judge is to make unbiased decisions. A judge engaged in ethnoactivism may conceivably be biased against a defendant whose political platform runs against those interests. This is simply not a racist claim. The mistake you and Ryan make is to misinterpret it as a much more general claim than it actually is.
You said there was an extensive list of racist comments by Trump, but then the only examples you can bring up are incredibly flimsy. It's therefore hard to take your accusations seriously.
It is an absolutely straightforward example of a racist comment. Trump knows full well that his complaints about Curiel are baseless, so he's lashing out by trying to make something out of the fact that the judge has Mexican parents. In other words, he's using racial insults because he's lost the argument. In legal and logical terms, he might just as well have said that Curiel is a poopy pants.
Quite often there are efforts to weave conspiracy theories around Trump's incoherent ranting. So in this instance, there is some kind of story about how the California La Raza Lawyers Association is a shady "ethnoactivist" group, whatever exactly that is supposed to mean. All of this is just a way of providing cover for another one of Trump's uncontrolled bigoted outbursts. The judge is a member of a completely unremarkable kind of law association. It is no different from a gay judge being a member of an LGBT bar association, or a black judge being a member of a black bar association. Everyone knows that these sorts of organizations have the purpose of tackling specific problems faced by (respectively) LGBT and black people in the legal profession. To say that membership of such an organization can disqualify a judge from a particular case is effectively to say that gay judges can't make judgments on gay issues and black judges can't make judgments on black issues. That would be racism/homophobia par exellence.
By the way, you may recall that Trump has expanded his comments on judges since. He also believes that he could not be judged fairly by a Muslim judge. I wonder what complicated theories people have come up with to try to justify that comment.
It says something quite extraordinary about the present state of the country that there are people willing to defend a Presidential candidate who attacks a federal judge using ugly racist language. Even mainstream Republicans like Paul Ryan don't want to touch this.