I don't think you're getting a straight answer from NCL. Southwest has no trouble accepting a one-way reservation - they sort of pioneered this concept, IIRC - and there is nothing in their system or set up that would prevent you from viewing that reservation, checking in online, and printing or downloading your boarding pass. I wouldn't be satisfied with that answer, even though I'm sure the thought of spending even more time on it makes your eyes roll back in your head.