I feel like the MC didn't mean it on purpose by the delivery, but yeah, not knowing what wasabi even is hearing the enemy that captured you say it'll be used for torture as a "prank" would actually be terrifying.
Umm, I actually like the prince. Being level headed is worth some points and he's revealing that his country is being idiotic or possibly puppeted. Story wise that's less interesting to me cuz we've just been told the antagonist is super small potatoes and actually really ****ing dumb on a national scale. There's actually another guy out there that's boosting the antagonists but an even other guy out there that's the supplying the whole ordeal. That makes the enemy we've been fighting off look much worse and sets up this potential long background campaign.
Character wise, yeah I'd like the prince to join the main cast. His bit would actually be fun if not overdone, and there's potential for development with him. Something like him learning about how the MC treats his citizens and the care and loyalty that relationship creates and how its his duty as a royal to create an atmosphere where his citizens feel safe and happy.