You've hit the nail on the head here. This is more or less the exact criteria and understanding we have of "fluency" in sociolinguistics and it is disappointing that so many others in this thread are trying so hard to come up with some neat little box they can package "language" and "fluency" together in. It reeks of 60's understandings of language and psychology (thanks Chomsky).

I'm especially happy that you brought up the idea that even native speakers are still growing in their own language. People don't realize that despite being fluent, they still have huge gaping holes in their knowledge of "their own" language. Show any common person a "lawyer-ese" document and their eyes will glaze over.