She does not abandon Narnia, but, as Animus recently described, "took a different, later train."

Oh, I like that phrase, it's very good; personally I always assumed that Susan wasn't ready to come back yet and wouldn't have seen Narnia as the rest did, and that would make things difficult.

I'd say you're doing a good job with making things conflicted; that is the true test of an adult story, I think, not whether it has adult elements but whether or not it succumbs to a less mature need for easy answers and monochrome characterisations. And I think that you're putting together a very strongly supported case for the impossibility of Susan being, as you said, frivolous and stupid; her experiences have made her a person that is well beyond that stage of life, so we're left with an incongruity. You let these characters be people beyond what would be easy for telling a story with, and they carry the story quite well.