All Actors always have an outer of a level, that is a assumption throughout many systems. The Actor (and Component for that matter) attachment system is independent of ownership hierarchy.
There isn’t a particularly good answer to that. In some ways the split is a vestigial remnant of the engine’s history. The World is, for the most part, the object you will interact with. It is very rare to actually read data or manipulate the Level directly other than the fact that the Level contains the Actor list. I might track down an old email I wrote about the relationship between Worlds, Levels, and Actors for someone internally and polish it out in to a Blog post, but generally, I’d suggest you don’t need to worry too much about the mechanics there.