Man, the problem with it is it is just a terrible terrible move. Like, I'm going to go out on a limb and say that the reason every ghoul/zombie in the game is clearly a human isn't because the modellers were terribly lazy, but because not that many non-humans became Scourge! Like all the dwarves/gnomes/elves killed and raised were probably so few in number they're already dead. So propotionately there should not be enough death knights running around for players to be them, let alone ones of ANE race they fucking want.
The reason there isn't many non-human scourge is because the Plague of Undeath was sent into human lands. Of course there are dwarf/elvish/gnome/etc scourge, just very few as you said (modellers were lazy). But, one doesn't have to be scourge to be a Death Knight, so this is a moot point you bring up, really. The Lich King twists the soul of the being, so they retain their humanity, (and therefore aren't Scourge) but now serve his will.
He'd take any strong hero with heroic power into such an army, as long as they can be corrupted to his side. Especially since he has people everywhere building up to fight him.
As for breaking free of his will, it's important to note that the Lich King has seemingly had his control wane by the time WoW starts. Namely, the Forsaken, broke away from his will. Most likely this is because there's possibly millions upon millions of undead, and he's finding himself unable to control them all (this is just a theory, it isn't really explained, yet.)
Most likely, the Knights of the Ebon Blade have found some magical god item or something that circumvents his will or some shit. I'm sure that reputation arc will explain it.