Except if you go into the character creation UI you will see two Pandaren options underneath the races selectors for Horde/Alliance, and we know there's a third that is also unplayable but appear in-game as NPC's the like. I am pointedly saying not to conflate the terms here and use the one that is applicable to what you're discussing - in-game "race" vs. ethnographic race. Conflating the terms is the problem, yes. In the case of the Pandaren their faction identity constitutes the in-game "race" choice you make.
Why is that? The Lightforged Draenei are definitely a great pick to underscore the differences between in-game race and ethnographic race, even better than the Void Elves since the Void Elves are more substantially changed both physiologically, ethnographically, and politically (turning into a Void-tainted species, altered appearance, and with a change in faction affiliation to boot). The Lightforged Draenei as still obviously Draenic in appearance, are Alliance-affilitiated like their regular brethren, and beyond an additional ritual to give them golden tattoos and eyes are no different than their cousins who arrived on the Exodar. And yet in the game UI they appear as another "race" altogether.
They were still wrong - their admission of that wrongness wasn't what caused the original wrongness. What are you trying to argue here? No is claiming that having been wrong before makes them wrong now, it simply means they can't be used as an objective authority on the matter - their word doesn't make them wrong anymore than it makes them right, it's just their opinion on the matter. Even Blizzard came right out and said "there will never be playable High Elves ever, from now unto the end of time because that is the substance of our decree" it wouldn't matter, and it wouldn't make them right. It's what would happen, sure; but people would still argue it because it's not a matter of objective truth.
Depends on the argument in question - there are different forms of argument all with different means and procedures for proving truth. Evidentiary argumentation is one form of arguing, yes; but it has its necessary limits. This is not a type of argument where evidence is paramount because it's a dialectical argument - it is about opinions and subjective stances, what is or isn't proper, what should or shouldn't be done. What Blizzard thinks is simply one position in that argument, but it's not the truth of it anymore than my stance is, or your stance is. It's like arguing about something that should be done in the future: how you cite evidence for something that has no rational existence? What's your evidence for where you should go have lunch tomorrow? Is pizza superior to salad, etc. etc. etc. Even arrogance doesn't make someone "wrong" in this kind of debate, it just means people are unlikely to go along with their opinion because it's apt to badly stated or spoken.
Their rationale is valid and perfectly worthy of citing, yes; what it doesn't do is make them automatically correct in the matter.