Assassin isn't really her ambition, just a transition state to make up for lacking strength. This happened before when she baited the criminal adventurer into a duel and explicitly lets him execute a full power maneuver to trade blows against.
Dungeons have already gone through that phase of copying the successful dungeon. Which is why some of them have considered hiring a consultant and became successful and thus our dungeon gambled their saving to hire MC.
It's not a credible "threat" from some random but a friendly "suggestion" from a city official that Mizuha would worship the ground of. The news delivery person matters more than the news here.
There's probably a lot of pomp and circumstance needed to get a legit continental summit going, way less is needed for schoolchildren to have a private get-together.
It's 100% on Katsuya for not issuing any orders. The chain of command collapsed and people started doing their own thing which lead to a lot of people being in splat range instead of maintaining their relatively safer original plan.