This one part is wrong though:
Whenever you have to brushes, no matter if they're entities or just plain old brushes, jammed into one another, it'll cause a leak.
Overlapping brushes will not cause leaks. Entities or not. Just look at your origin brush.
The only way to cause a leak is if an entity has access to outside your map, assuming the entity can move around and pass through anything but regular, non-entity brushes.