According to your very own definition of a quack ("someone who doesn't use the right process, who avoids peer review, who insists they can't be wrong"), Pauling *was* a quack.
Yes, he had two nobel prizes in a completely unrelated field. Just because he was an expert in that field does not mean that he is an expert in all fields, nor does it mean that he even felt he should use the scientific method in other fields. (Obligatory SMBC.
There had already been studies showing Vitamin C didn't cure colds before Pauling made his claim. His (completely anecdotal) claim made other people do *more* studies which also showed no link between Vit. C and curing anything. From the article: "Although study after study showed that he was wrong, Pauling refused to believe it, continuing to promote vitamin C in speeches, popular articles, and books."
Yes, he was a smart guy and brilliant in some ways. But in medicine, he was a quack, by your own definition.