Thanks for the feedback! I see the discussion you're referring to here and I will have to go take a closer look!
To answer your question about Labels, they don't actually need to be unique. That is definitely something we can add to the Entity Cheat Sheet, along with more tips about labeling strategies (detail vs. privacy). The only computation that happens with the label within Central are checks to make sure it is not empty.
At the moment, we don't have a mechanism for indicating that a specific property should be unique or have Central enforce that. It's something we are thinking about, though! Currently, the only enforced unique part of an entity is the UUID, and this can't really be specified by the user.
For your last paragraph, are you suggesting a section of the cheat sheet that could show a small example of this -- of filtering an entity list by several levels of cascaded drop-downs? That could be added! I'm also working on something somewhat related, of making a tutorial of using entities to represent nested levels of geographic information. You've given me the idea to expand upon that and show how to filter entities by properties selected from other entities.