In fact, the idea of Death's garden partly comes from a story by the Danish fairy tale writer Hans Christian Andersen: Historien om en Moder (The story of a mother), although there Death has a hothouse, and each plant represents a human and is replanted in Paradise when the human dies.
I just liked the idea of Azarael having somewhere beautiful and comforting to relax, and I think Diego may be the first person to be invited there.
I'm not sure it would be against proper protocol for Ayil to issue Theo with the task of checking Diego and Azrael out, but he hasn't had time to do so yet.
I want to involve Angel and Imp, but, like you say, perhaps they won't be needed?