The "presently no wars" part is quite literally a "calm before the storm" type of situation. Like the couple decades of peace between WW1 and WW2.
And to point out - these rules don't work. Wars did happen in the past, during Queen's rule, and are going to keep happening in the future.
War is always going to happen, you cannot stop war, especially with such disparate species all living on the same continent. The MC on Croix's path just got a taste of that with the vampires- it's not like IRL where everyone's human with mostly superficial differences, you've got vampires that can't help but drink every last drop of a person if they find them too delicious, it's small wonder they're treated with suspicion and caution.
And yeah, wars happened even after the Guardian decree- they continued to happen after the Geneva Convention too, but I think we can both agree it's better that people aren't having their flesh melted off their bones in trenches IRL, just as it's better that one nation can't team up with another nation and then mobilize to wipe out another nation without any warning in-setting.
I'm not saying she's perfect, because she aint- I'm not even saying I like her, because I don't- but what I am saying is that it's easy to criticize and say "do better", but ultimately that's a nothing statement because it says nothing, it contributes nothing.
Here's an example, something I would think the queen could do to better benefit neutral kingdoms is this: send undercover ambassadors (so normal size) to neutral territories, ones who can defend themselves, and inform all the other kingdoms of it- say the kingdom of Timflame is investigating a matter in neutral territory undercover, and are not to be disrupted. On paper this doesn't sound like much, but that's where politics come in- now suddenly, if a giantess attacks, Timflame can claim that their citizen was assaulted and can legally take action against the offending giantess.
Now unfortunately even if that idea was put forth to the queen she'd turn it down because she's a bitch, but that's a point of fair criticism to make against her, not just "um there are wars between these incredibly unalike nations with nigh-opposite cultures so she bad".