It could take months to get them to shoulder the responsibility, but they will. You have the proof. File with the BBB and ask about Lemon Law procedings. Take your time, it's your money. Don't expect them to do a thing though, I know from experience.
Some vehicles experience hard start at high altitudes and cold conditions.
As you can see, this problem is well known. Write Laurie Bembinster, or anyone for that matter, at National Consumer Affairs and ask the same question regarding your concern about the Xterra engine and you will receive the same answer. It could take months to get them to shoulder the responsibility, but they will. You have the proof. File with the BBB and ask about Lemon Law procedings.