There are multiple hourglasses so each of the countries can tell when the wave is near. There isn't really more reason than that but makes sense so the countries can prep their soldiers and subjects.
Originally, the major countries were supposed to summon one hero. We aren't sure which would get which hero, but the demi-human country was pretty much guaranteed to get the Shield hero, well, up until the King fucked that plan up by summoning all four of them while the Queen was away negotiating everything. He pretty much did it so he could get the shield hero to fuck with him while denying the demi-human country of help, hoping they would get weakened by the waves. That was also why it took so long for the Queen to appear. She was making sure her country wouldn't get fucked by all the other ones for this act of stupidity.
The wave is technically happening all over the world, but the heroes get sent to where the invading monsters are most concentrated. Though, I could be wrong. It may also be the scenario where they get sent based on which hourglass they are closest to. This is the one where I just don't remember well.
And the waves are happening because the world itself is unstable. TL
R, the world was originally 2; one with only humans and another with only demi-humans. They got smashed together to make one world, and the waves are a brute-force solution to restore order by destroying said world.
The turtle is basically able to collect all the magical energy of the beings it has killed and use itself as a dimensional stabilizer. If it is able to kill enough, it can effectively keep the waves from happening again. Technically, it isn't infinite, but it is close enough. The last heroes sealed it because they didn't want to use such a strategy if there are other ones still on the table that are possible, but have it available as an option if there is truly nothing left.
I would say it never truly reaches the point to what isekai is like now... but, it does go back to the overall theme of good vs. evil inevitably. This is the scenario where all the people in the world eventually band together because they have to choose between fighting each other and guarantee they die, or band together so they might live to see tomorrow... when they technically are free to kill each other again. Even if that happens, it means they have a future again.