the uncertainty ... while native speakers are usually not even aware that there is an issue here.
It seems you are talking about being self-conscious, not about language fluency.
The one was that it's not entirely clear that there aren't any such in-principle impossibilities
Why in the world would there be "in-principle impossibilities" -- where does this idea even come from? What possible mechanism do you have in mind?
only the most exceptional L2 learners can in practice expect to ever achieve native-like fluency.
Well, let's get specific. Which test do you assert native speakers will pass and ESL people will not (except for the "most exceptional")?
It seems you are talking about being self-conscious, not about language fluency.
I didn't say it was about fluency. But I don't think it's about self-consciousness, either. Native speakers of a language pick the appropriate tense and aspect forms of verbs perfectly effortlessly - or how often do you hear a native speaker of English use a progressive in a case where it strikes you as inappropriate and you would say that they should really have used a plain tense here, for example?* - while for L2 speakers, it is generally pretty hard to grasp all the deta...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.