but any number of scenarios of revelation could have been consistent with his & Elizabeth's characters. Darcy didn't decide to write a letter, Jane Austen decided that Darcy would write a letter. Why did she choose to make the disclosures in that compact way?

The early part of the letter is Darcy's pride explaining itself, but the accounts of the Darcy/Wickham matters could have been JA's own narration. I think the letter format allows the author to narrate without narrating.

In addition, this letter becomes a character in the story as it moves forward. It is a tangible object. It burns itself into Elizabeth's consciousness--at first she wanted to discount everything it contained but found herself incapable of ignoring it to the point where she reads & rereads it so much she nearly memorizes it. Her consciousness of it informs the succeeding chapters of volume 2 at least, where the action is informed by it and illustrates it.