That's an interesting theory, though I don't think the Wishing Tree has that much significance (though maybe it does).
I personally believe the first sightings of the fire gods and feather and everything past Dulcie's first meeting with Cameron till Cam dies is just a hallucination, and Dulcie is a biblical angel. First of all, when you think about it, nothing makes sense/adds up: What is the significance of Gizmo attending? How did the band disappear and appear with no cares given from anyone? How does Dr. X connect to Dulcie? How did Dr. X accomplish what he did? How did the jazz musician live (wasn't he dead?) How does a yard gnome talk? I think it's just a hallucination Cam created, using bits and pieces of his memories, for example the gnome being a Norse god and his mother teaching him Norse mythology, Junior, or Gizmo. This continues until he faces himself and promptly dies. Now Dulcie is more of an interesting character. She's a messenger, probably sent from God, to inform Cam he has two weeks to live. Note how Cam's friends or any of the other characters don't notice Dulcie, and how Cam always goes on about her details... it's because she isn't part of the hallucination: she's real.
Unfortunately, my theory can't explain a lot of what's happening :/
My favorite character is the yard gnome.