1)I've never heard of a Worldcube, and I've got a working map. Unless you mean a skybox, you don't need it (and you only need a skybox if you plan to have a sky).
2)You'll know if you have a leak when you bsp your map. There'll be a big ascii sign in the dos window that says leaked, and after that, in the editor, a big red line will follow along the path from the closest light source to the hole in the map, allowing you to find it no matter how small it may be.
3)It's true that using the hollow command isn't the best idea all the time. It's better to make each wall individually, either already in position or moving them into position after you make them. If you want all your walls to look the same, you only need to make one, then use the copy/paste brush commands to recreate it (the new brush will appear in the exact spot of the old one, assuming you didn't move the old one after you hit copy). If you want to do it the easy way, it's best to remember that people will usually only see the inside of rooms, so as long as the inside looks lined up correctly it doesn't always matter how the outside looks (though if you're like me the outside is usually as neat as the inside for disk space purposes and to save download times on the internet).