how much of the narration is omniscient "narrator" voice and how much is in the characters' heads. I feel like the narration's a bit more distant than what you'd generally see in a Western novel, and that some of the ruder observations are actually the author's POV seeping in

Yeah, it's definitely an omniscient narrator, not the kind of close third person POV you generally get in Western novels, and a lot of the snarkiness is the narrator's voice (not the author's). Priest has a great talent for snarky narration! But since the omniscient narrator dips into different characters' thoughts, the boundaries between what's narrator and what's character are a bit blurry, and it's sometimes hard to say whether a specific snarky remark is the narrator's or a character's POV.