I don't think they are unified in the idea thou. Inviting Serie to the day of the country's founding can be a sign that relations with the other countries is about to warm up soon. Assassinating her won't be the Empire's casus belli but it would be for the Magic Association and in extension, the central continent country.I mean you could see it that way but i think that the empire is simply preparing itself to expand territory, the demon king is dead, the demon army may still have some soldiers but they seem like a shadow of it former self, so the empire probably wants to expand now that they dont have to protect the whole world, and serie is the biggest thorn on their plan.
that makes no narrative sense,I'm hoping the merchant actually managed to bluff Frieren instead when confronted and avoid a full on battle for now....
Not that the trio would lose or anything, but I kinda wanna see more scheming before any big battles happen....