Billy, wonderful character as he is (with what is quite possibly the best name in the world), can be hard to get a handle on - at least, I find it so, given how reticent he is. You've definitely got him wrapped up, though; when I read this story, I think, "that's Billy", with never a question about it. Jesiah, too - even if it's only the Jesiah in Billy's memories - is presented brilliantly.
I think, though, that the true measure of the story is thus: in the paragraph just after "Fiat justitia ruat caelum", I actually exhaled a held breath. Which, I think, means something that could be typically translated as "oh man so cool", with anything up to eight exclamation marks.