1: put those places somewhere else. Anything behind it will always show.
2: Used for blocking vis, such as preventing the contents of a room from being drawn if the door is closed.
3: Structure brushes are used to calculate visibility and such, detail brushes are not. Think of detail brushes as not really being there and you're on the right track.
4: It's used when you want some, but not all brush faces textured with something else. The brush doesn't exist in the game, or rather, is Not Drawn.