"So if I’m understanding correctly, everything that could possibly happen is happening simultaneously with every action in the universe. And not only that, but it’s happening in the same physical space!"

You are not understanding correctly. This is not what Carroll or Everett are saying at all. They're saying when a quantum measurement occurs with a coherent quantum system, causing it to decohere, the universe splits into different worlds. The *size* of those worlds in Hilbert space is proportional to the amplitude of the inverse of the wave function squared. That's what it says. That's not the same thing as saying, whenever you go to a restaurant, the world splits into one universe per item on the menu, or combination of items on the menu, with a different world where you order each thing. This is applying very classical reasoning to try and understand Everett, and it's leading you astray.