So here's where I'm coming from for this situation. Feel free to discuss why anything mentioned here is wrong.
Foundational Theory 1: The pirates are here for the Kurokirimaru.
We ran a sweep over the ship before departing and nothing unusual turned up. As a character Nazrin has every reason to believe there's nothing special on board aside from the Mouthful Blade. As a player I'm going to be massively annoyed if the game was planned to go in a way such that the PC's defining character trait just up and crapped out at the most important point in the game (one of the two most important points, at least) for no reason. Trying to account for something else being on board is completely pointless and irrational as far as I'm concerned.
Corrolary A: The Blue Maiden Crew are innocent bystanders.
This follows from Theory 1 in a pretty obvious manner. That several of them have expressed lack of knowledge of any such relic lends itself to this fact regardless.
Foundational Theory 2: While their execution is sorely lacking, these pirates (or at least the higher-ups) truly do try to hold a moral higher ground and keep a higher behavioral standard than your average pirate.
This one is slightly more metagamey than Theory 1, since it's a Chekhov's Sentiment situation. I don't see why Ren (why is one of the lieutenant pirates named 'zero'?) would insist on this so much if it weren't true. From a character standpoint, it's probably true because we're alive and allowed to talk with the pirates, Sotsu constantly gets admonished for her behavior toward us, and frankly Naz might as well act under the assumption it's true because she has nothing to lose - if it isn't, then there was never any hope for the Blue Maiden to begin with.
What comes from these two theories is that, once we're in Aya's presence, I believe a guilt trip solution presents itself. The pirates may claim this higher moral ground, but their objectives were going to kill us regardless, and they just hacked their way through a bunch of innocents to boot. Aya needs these things pointed out to her; once that's done, I think a war of words is very winnable.