Amerykanie juz maja tak na stale.
W AmE powiedza I just did it w kazdym wypadku po just.
No. That's not true.
Generally it is true. For a student taking TOEFL exam, using PP after "just" in examples such as discussed above, will be considered a mistake. In everyday AmE it's a dominant form also. In extra polite and far fetched situations PP might be used, because like everything else in this life, language has its fluidity.
For the rest of your sophisticated explanation, you are truly convolutedly and astonishingly flabbergasting me :). Reading your explanations on grammar can be quite entertaining, but hardly useful for intermediate level students. If you really want to help, you need to ease out.
I am easing up.
Your generally (formerly always)
I don't think that for a student who takes a TOEFL and uses the present perfect tense after just
instead of the preterite would be a mistake as long as the event time is not made definite in the same sentence.
If you misspoke, correct your post and say " I am sorry, I misspoke."
Second, people are not convoluted. Concepts and ideas are.
Third, your English isn't intermediate level yet.
Fourth, I'll have you know that in AmE I have just done it
there is an underlying hint of accomplishment, often in retrospect, and only apparently contrary to the immediacy of just