I definitely agree with your broad point, especially about headline grammar. I disagree on this being mostly a native vs non-native speaker issue, though. I'm sure you will agree that a language where most words can be used as multiple parts of speech is more ambiguous than one where they cannot.
Oh, like I said, I'll totally grant that if we're talking about lexical ambiguity. Words in isolating languages accumulate extra meanings, just like morphemes in inflecting languages do.
But again, that's just because of highly lexicalized grammar; it's practically no different from having an "ambiguous" system of inflection, or bizarre energy-focused polysynthesis, which a native speaker would never mistake. That's where I'm saying someone who grew up with a strongly inflected or polysynthetic language would be biased.
And I hope you don't take this as a criticism of your English— you're obviously perfectly fluent! There's simply a basic level of intuitive understanding of a language which is impossible to get without growing up with it from infancy. That's why in linguistics we don't talk about fluency as much as native vs. non-native speakers. It's where you can really separate the meat from the bone, if you will.
