Death of the author is not really something you can believe in. It's a foundational truth of literary analysis, and you cannot interpret any text without it. Following your logic, you cannot have opinions about any type of art without verifying it first with the author. Any modern art critic nowadays will tell you that you simply cannot analyze art based solely on intentionalism. Especially if you take into account many authors include messages in their work subconsciously.
You are really just repeating what you already said, and I'm not sure why. I already gave you an answer. Rumiko's intention as a creator is irrelevant to the reader's interpretation, particularly because her storytelling is contradictory and full of holes that make it very obvious she never actually studied dramaturgy.
As I said, taking yourself out of the equation to reach "rational" conclusions about art makes no sense. Art is subjective. You will never reach the correct, most rational interpretation. It's an exercise in futility.
Inuyasha being written as a racism allegory makes perfect sense, especially taking into account Japanese imperialist history and tradition of following hierarchical systems. The reason why you're having a hard time wrapping your head around that comparison is that you are taking things too literally and don't stop to consider the core concepts.
Inuyasha isn't a racism allegory in the sense that Daiyoukai represent, let's say, white people and humans represent brown people specifically. That's reductive and not what I'm saying at all. He is a racism allegory because he is a person that is neither one thing nor the other, and is viscerally hated by others as a result. That in between state is a mixed-child experience by definition.
Humans and Daiyoukai other one another based on preconceived notions that we as viewers know aren't true, or at least not entirely. "All Daiyoukai are savages that eat people," for example. That is the rhetoric used by Spaniards who to this day justify the destruction of the Aztecs. The same can be said for the statement, "Humans are inherently weak," when we know powerhouses such as Kikyo and Midoriko existed. Any statement that goes along the lines of "x group is inherently y trait" implies the act of othering, which is, surprise! The foundation of racism.
When you stop thinking about specific scenarios and focus only on the core concept, it becomes undeniable. And obviously, I am not saying that this makes sense "in the text". It's a given that I am speaking about subtext.
I am not saying that the Spanish dub is directly affiliated to Takahashi. What I am saying is that obviously, when looking at the source material, the people responsible for translating Inuyasha to Spanish saw a racism allegory as well. So they decided to use a word that has a strong historical weight in Latin American to refer to Inuyasha, when they could have chosen something else. So I think the opinion of professional producers who are fluent in Japanese and understand the original material in a way neither you nor I do is valid, and certainly not "limited".
I do not have a theory, and I am not bringing you evidence. I am supporting a subjective interpretation that is as valid as yours, and I think that's the point that keeps flying over your head. You do not need to agree with my opinion, but you also do not have the right to say it has holes, or that it makes no sense because it does. So much so it affected the version of Inuyasha that was aired in all of Latin America and other Spanish speaking countries. Which, perhaps you will find interesting to know, surpasses Inuyasha's fan base in the USA by far. So you might argue that version is more widely spread than the American dub.
To summarize and make it simpler, what disgusts me about Sesshoumaru as a character is not that he is a white supremacist in text. It's that the logic behind his hatred for Inuyasha and humans in general it's the same logic used by white supremacists. And without a proper redemption or repentance, that makes him undeserving of any type of sympathy by principle. To me.
