Personally, I think Amidonia's actions are quite sensible, given the setting.
An enemy civil war is indeed the best moment to strike. Especially if they, like everyone else, suspect even more parties will intervene to screw Elveniden. If they win, they'll solve the starvation problems and deal a severe blow to Elveniden. What about the moment the civil war ends? Well, it's true that attacking a fellow human kingdom won't sit well with their pact allies... but as they say, it's also true that Elveniden didn't sign the treaty. Amidonia did. If Elveniden declares war to recover their lands (who they actually stole in the past), Amidonia can in all justice demand that their allies fulfill their obligations and support them against a non-member party. They even enjoy popular support, a must!, since their economic problems have a lot to do with losing land to Elveniden in the first place.
I don't know the LN, so I'm probably missing details, but the plan, although risky, isn't stupid. Just too ambitious. If anything, I don't like that the story presents it as "Ha, look at that stubborn, stupid king. Doesn't he magically realize he's going against a manga protagonist, and thus the story says he'll lose?" from the get go.