You can make a tag for any notable work. Chronicles of Narnia would be a fine subject.
I've about George MacDonald's works with some success. I think Narnia would be on topic.
However, I'm a little hesitant about your first bullet point. I don't think we should be concerning ourselves with all the nautical terms used in the Voyage of the Dawn Treader ...
Where to post question about Christian theological implication of the Narnia stories?
Go for it on main site at Christianity SE. I believe it would be quite on topic.
Here follows a sample question on The Lord of the Rings:
Who symbolizes Christ in J. R. R. Tolkien's “The Lord of the Rings”?
If it were me, I would avoid asking the same question on ...