So Angel and I saw a while back an orc monk that was apparently also a Death Knight. …How plausible is that even? Is there any class that a DK could conceivably dual class as? (My vote is no but I’m sketchy enough on lore that I don’t know.) ?

I’d say no. Once you’re killed and raised as a Death Knight that’s pretty much it. You lose al the skills that you had as whatever class you were before and all you have are the abilities available to death knights.