We don't know that Simon's a digger per se, but more of a wandering sage. He's basically taken the Ender Route, because after being essentially a king he knows that having glory and power doesn't mean one has happiness. He may have gone on to do more great things in his time (hence the spiral eye), but we don't know about them (yet!)
Ahiru on the other hand is, well, a duck. It may be a theme of recognizing what you really are, but honestly I don't quite get the idea that she's happier that way. She's actually making a sacrifice of her happiness, to show Fakir that he doesn't need to change anyone's story (because if there was ever a place to start, it would be hers). She's pretty much sacrificing herself for the happiness of others, in that Fakir won't become another Drosselmeyer.