The timeline is not an object. Au Ra are not treated unusually if you start the game as one - despite the fact that, canonically, they should not be there or be an extreme oddity. Yugiri still hides her face until after "The Parting Glass" even though you're the same race and nobody is freaking out at your appearance. Etc etc.

A more noble Garlean being a Warrior of Light may be unusual, but it is not outside the realm of possibility. With the revelation that a small fraction of Garleans can wield magic, it is now plausible. It's still extremely, extremely improbable, though.

Death Knights became playable in WoW because they are the edgy anti-Paladins of the setting. Demon Hunters for similar reasons, though helped by Illidan's popularity in Burning Crusade.