Indeed, to keep it consistent with the achievement, it is a good and clean idea for the stars to indicate the highest difficulty won filling the prior "grey" stars/markers up until the difficulty achieved. As for the widget, it's a cool addon to it.
Regarding the alignment of the stars being in a straight line and smaller, inputs a cleaner aspect and not bloated up, which is indeed the feedback that a player should get from the map/UI.
As i mentioned before, the dots were just a placeholder to describe the idea (which was put together in a rush).
While the dots are smaller/cleaner on the other hand the stars themselves appeal more to the players subconscious of encountering them in other games making it much easier to comprehend and know what they represent, especially when they are positioned above a map selection point. I mean i myself encountered stars as indicators to a mastery of a Map/chapter/level or what other terms different games use.
Overall from the picture you posted, everything seems clean/smooth (positioning, size) and at the same time very easy to understand what's going on on the screen when looking at it.