Isn't she awesome? The more time I spend with her, the more she becomes one of my favorite characters of all time. I think thestrals aren't "dark" creatures per se - it's just that their magic is very solidly linked with mortality, which is just a basic, neutral biological/psychological trait that Luna has and Aziraphale lacks. Crowley wouldn't be able to see them either. Or at least that's how my thought process went.

I think someone with a duck daemon would be a person who's generally most comfortable and cheerful swimming on the surface, but can fly high or dive deep when s/he has to. Which kind of does suit Crowley.

As I've said, the third was the hardest to write - but I guess it does work, doesn't it?