Why is it “a defeated Napoleon, not “the defeated Napoleon” who rode off the battlefield and into exile?

This is a difficult construction to justify; but before tackling that thorny question, let’s pause to confirm that it’s idiomatic and quite standard, with a couple of other random examples:

A thinner Kim Jong-Il at the Supreme People’s Assembly — The Independent

A young Cary Grant plays her leading man for the second time. — Wikipedia

This construction is used typically to put emphasis on the adjective. My best shot at an analysis of “why” would be: because it’s evoking the spectrum of all the different kinds of Napoleon that could have left the field — triumphant Napoleons, preoccupied Napoleons, crazy cyborg Napoleons — and so it’s picking out a defeated Napoleon.

But this is very much a post-hoc analysis; as you show, one can also argue cogently for why the would make more sense in these places. Ultimately, this may be one of those constructions that just is — it’s difficult to analyse, but it’s part of how we speak and write (and has been for long enough that it’s not stigmatised by prescriptivists).

To my ear, incidentally, …the defeated Napoleon… and The young Cary Grant… would both also work fine in our examples, but The thinner Kim Jong-Il… would be incorrect.

It's a literary/oratorial device. You certainly wouldn't often come acrosss it in normal speech - only somewhat flamboyant prose or speechifying.

Think of it as meaning that Napoleon is a man of many aspects, victorious on other occasions, but defeated this time. Using the article implies that this is only one of several possible Napoleons that have or will exist. A writer might indeed use the as OP suggests, but a is more common for this usage.

I think the is more likely to be used when there are known distinct "versions" of the subject (in practice, often only two). In OP's example the writer probably doesn't see Napoleon as a simple man with only the aspects "winner" and "loser"; using a emphasises the complexity of the man better. In general, the tends to be used only when the particular aspect of the subject that's being presented is already familiar to the reader/audience.

The indefinite article "a" emphasizes that this is a new Napoleon, not the Napoleon that we were familiar with before.

I would not expect to see this usage colloquially. I am not sure there is a word for it, but I'd say there is a bit of license, poetic or otherwise, involved.