It’s just a framework for evaluating how people learning languages stand.

Most native speakers would be hard pressed to be certified as C2 in their own language. I think a lot would fail C1 because they don’t know/use some of their language quirks which would be evaluated. I know for a fact that I can’t properly use some modes and tenses in my native language without a rule book.

Is the 'beyond C2' defined? C2 is the highest possible grade in the Common European Framework of Reference for languages. How would one ascertain that someone is beyond C2, given the lack of generally accepted criteria?

Maybe if you can create professional-level works in that language? Ie poems, lyrics, prose, etc.

It's just a certification level that is almost meaningless compared to the natural Version of the language. And with some native speakers you honestly wonder why C2 requirements are so sophisticated.

Sure they can. It is just a matter of immersion.