Livre d'Or

That's a really interesting perspective, from a religiously knowledgeable Christian child presented with the stories as being about your own religion. I think that's probably the audience Lewis had in mind, as he was writing in an era when he could reasonably expect most children would have a fairly comprehensive Christian education. I don't think he was intending to bait-and-switch his readers by presenting an enjoyable fantasy story and sneaking Jesus into it.

The Atonement stuff kind of offended my sense of justice when I was a kid, and I can't say I'm entirely over the feeling that it seems deeply unfair for God to punish all of humanity just because their progenitor committed a sin, and equally it seems deeply unfair for God to punish sinless Jesus for the actual sins of anybody else. It's really interesting to me that that's something you struggle with.

I think Dante, too, would have been influenced by Classical Pagan thought, so it's not surprising you see similar elements in Dante and Lewis. And I do think you're right that apocalypse literature doesn't fit very well into the children's fantasy frame, there just isn't a way to do it that would make it work within the established genre.