Doesn't matter. Original sin is not about whether you're a bastard. It's about whether your ancestor being a bastard condemns you to a shortened life of toil and pain. Elves are "unfallen." This is why they can return to Valinor and why they are greater than Men. They can do terrible things and make mistakes, but they do not bear an inherent burden of sin.
In the interview that you generously quoted, Tolkien says that the elves are not eternally immortal but just unthinkably long-lived. Further, I still maintain that if the elves were "unfallen" as you say, then they would be incapable even of venial sin without recapitulating the Fall, which would then potentially pertain to their descendants, etc. Adam and Eve did not sin before the Original Sin, now did they?
They are still not stark white man-grubs.
I've got my copy of Sturluson right here (Faulkes/Everyman/p16): "The dwarfs had taken shape first and acquired life in the flesh of Ymir and were then maggots, but by decision of the gods they became conscious with intelligence
and had the shape of men though they live in the earth and in rocks."
Emphasis added. So I'm afraid that you're wrong here. It took me only a few minutes to look that up.
Listen to him:
*SNIP*
The Wikipedia reference does in fact come from reputable Tolkien scholarship. Basically, lots of people have known this about Middle earth for a long time, and it's utterly uncontroversial. The controversy is more about what that says about the subtext of the work and Tolkien's character, on which points I'm inclined to be charitable. The main letter which is typically cited is #176.
Thank you for the reference. The interview speaks of the dwarves still principally in terms of their language. It's a good source for your position, though not definitive.
Letter #176, which you also graciously cited, says this: "I do think of the 'Dwarves' like Jews: at once native and alien in their habitations, speaking languages of the country, but with an accent due to their own private tongue...." That's all. Hardly definitive. When I think of the Jews I think of things like "Chosen People", "history's standardbearers of monotheism", "victims of unjust persecution" and so on. Tolkien seems to think of them primarily from a linguistic point of view... which is unsurprising. But "dwarves as linguistic Jews" is different from "dwarves as thematic Jews", if you follow me.
It's not petulance when you're right.
Rubbish. For one, there's nothing on any of your main points that definitively suggests that you're right. You have unilaterally decided that you're right (a lot like Wikipedia, maybe). Second, and more importantly, being right or wrong is irrelevant to civility. Allow me to suggest that you consider whether the attitude "I can be rude as long as (I have decided) I'm right" is really a mature attitude, or is something more like a child's attitude. I expect civility from you even if you're announcing your paragraph-length solution to Fermat's last theorem.