I think that, at this point, what riverkid and I disagree on is the abstract matter of why native speakers use finished adverbs with the present perfect: he formulates a rule and calls it an "override", while I'm suggesting that native speakers modify their utterances in mid-speech and so produce non-standard structures. On a real practical level, though, it boils down to the same thing: native speakers prefer not to use finished adverbs with the present perfect, but sometimes they do.
I am a little surprised, though, that your teacher says the structure "since ... ago" is not allowed. It's extremely common and not that difficult to explain.
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