Update: when I dropped the Xterra off at the service center Thursday afternoon I discussed the problem for several minutes with the service writer. As I carefully described the symptoms he explained, "It's probably [part 1] or [part 2] given [the specific combo of symptoms described]."

That gave me optimism the repair could be completed Thursday rather than stretching across multiple days. It also validated my idea that taking the car to a dealership would be good because the mechanics there would have more experience with the model's engine and control systems than the mechanics at a generalist shop. It's Thursday approaching noon now; I expect to get an update soon.