This a Coen bothers film---of COURSE the joke's on the viewer.
I agree: When Moss is taken out, off-camera, it's a huge cheat. The rest of the film was pretty much crap, excepting perhaps the confrontation between Anton and Moss's wife.
I know that some people will scream that that's the way the novel goes, but since I never read the novel (and now never intend to) I can only judge it as a movie, and frankly any movie based on a novel has to stand or fall by how it functions as a movie. And one conclusion I've reached about the Coens long ago is that they are incapable of simply making a "good movie"---they can't help playing silly, self-referential, post-modern games. Fucking around, in other words. But they've got two-thirds of a good movie here, which is really great for them.