We used to have a calendar where people could bookmark lessons for "publication" into their account individually. And I can look at adding it back (the functionality is still there) but it was not very widely used and I would be surprised if even one in a hundred people even knew that it existed....
What happens now is that we add all of our lessons to people's accounts by default, scheduled by their original date of publication. Lessons get tagged as "studied" as people use them so there is some way of sorting the material people have looked at from the archive of stuff they haven't seen yet. If this is what you need, try clicking on the "banner" link at the top of the lesson archive page. It will show you a list of all lessons in your account, with simple tags (studied/unstudied/all) you can use to navigate through materials you have and have not seen.
Are there better ways of doing things? We're very open to suggestions. One of the big usability problems is actually making it obvious how things work. We've had more complicated setups, but they've always involved putting a lot more explanatory text on the site that no-one reads. When we added a more limited set of lessons to accounts on account creation and required people to manually add levels and shows, we would start getting emails with questions about where all the lessons were, for instance.
As far as the question of implicit structure goes, we are putting together a more structured set of lessons for people who know absolutely nothing -- i.e. "your first ten lessons" somewhat along the lines of the intro series we have for Cantonese. The pedagogical problem with making other lessons hierarchical is that the more anyone knows, the less a one-size-fits-all course makes sense. This is why we bias towards rough levels with 10-minute lessons that people can push through at their own pace and skip if they find them too easy. Suggestions on features and things we can do to improve the site are always useful though, and -- as promised -- we are always happy to refund anyone who isn't happy with the way things work for them.