I have to say I can put your cited examples with other experiences I have had in the past and it does seem to fit the criteria.
I have compared myself to other Asian-Americans before, in the manner of our daily use of English and syntax. Your research makes them valid examples of the language proficiency thing.
I noticed that for those who emigrated into the USA after a certain period of time - be it during around Jr high or after (high school FOBs had the worst accent/grammar structure JMO).
For example, I recall these for these friends, all of Chinese-Taiwan ancestral heritage:
Friend A: Born in USA (like me)
Friend B: Born out of USA, immigrated at age of 4
Friend C: Born out of USA, immigrated at age of 10
Friend D: Born out of USA, immigrated at age of 14 (some English education overseas, but was not main language.)
When we grew up, we all ended up in the same high school. It was by then I was able to start distinguishing speech "inhibitions" or whatever that set them apart from my English.
Between A and B we always got along fine. There are very little to note in speech disambiguation. This supposedly explains where we have learned the language, harnessed and fine tuned it for our uses.
For C, I had noticed that he could not grasp the context of some form examples as you have stated. "I need to go camp in the park next week. Can you watchs my dog?", "Yes! Last week I was able to finally kills the boss!" But often simple mistakes like these are heeded if they are not written on paper, we give a ? at first but then understand it.
Friend D just simply.. did not ever make sense when speaking. The common stereotype goes for us Asians who cannot speak properly, "Fragmented English", with a taste of accent.
That was how bad it was to understand him. However in written proficiency he was able to do fine. (I suppose this is where it draws the acquisition trait). It was almost as bad as first generation asian parents, but he was able to make more sense than them. But his writing was WAY past their level. When I applied a combination of Chinese + English grammar together, it made his speech more understandable.
However, turn this all around and play the foreign language game.. D and C simply destroy the rest of us at speaking our native tongues, hands down no contest needed.