K-1 cases go through NVC only to get a DOS case number assigned. No documentation is required. NVC holds them there until the consulate requests the cases. Then, at most consulates, the applicant schedules their own interview. There are a couple exceptions which come to mind.
BTW, unless things have changed, all PI immigrant cases, by law, are automatically given preference at USCIS due to a hurricane years ago. I'm not sure if that is still the case.