I think Rabadash's ending is supposed to serve as a warning. One of the worst things you can be as a Christian is a fool. A fool is someone who gives in to their personal desires instead of practicing restraint and following God's word and fools are often destroyed by their own sin. The Calormens are not very smart. They quote silly poets, blindly follow a corrupt system, and praise extravagance. The word ridiculous is often used in association with Calormens and Rabadash. Rabadash let his pride and foolish ways get in the way of his potential salvation. So, he was turned into a donkey, an ass, which is another name for a stupid or foolish person. Rabadash now fully embodies foolishness and he is trapped in the religion and world that made him such a fool in the first place. I believe Rabadash is meant to teach readers not to be a fool or follow any foolish religion or else they'll be trapped forever.
I think this is absolutely correct! In fact, there is a story in the book of Daniel in the Old Testament that talks about how king Nebuchadnezzar (the one who tried to burn the guys who wouldn't bow down to him in a furnace, but they were saved by God) into an ox for being so prideful about his kingdom. Serving a prideful person with a good dose of humility is the ultimate justice in the eyes of Christianity and much of the Western world, and Rabadash certainly fell under that category.
excellent connection!
Ah, I really like this! And I suspect you're right; this feels very much like something Lewis would do. The only thing I'd want to add here is that this point ties in in fascinating (and troubling) ways with our conversation about orientalism. The connection between the fool, the unhuman (here a donkey), and the Easterner, are really suggestive of the dehumanization inherent in Orientalist thought.