Yes, yes and, pretty much yes.
The last one is kind of dependent on how you want to work with it as a GM, really. I do have a one-off adventure that is it's own realm, for everything else they are more fleshed-out more. I have a Realm for an entire campaign book, for example, and another that is for an entire, very large, regional setting. It really depends on what you want to focus on but arguably you could go as large as you want in any given realm just keeping reference stuff in the World Almanac and things you need for the current adventure in Story. It's a snap to move them between the two to keep Story as neat or as messy as you like.