plnrns xaritos: just as I thought. The Greek would render it as a combination of "full" and of "grace." Is there an alternate reading that has "plnrns xaritos" in the Luke 1:28 passage? (I don't have my text with me...it's still in shipment from Oklahoma...I recently moved.)
As you say, though, if that "plevnw" language were to result in Mary being "immaculately conceived," then it would also have to apply to the Saint Stephen.
What we have, then, is what I orignally stated: the only argument for the "immaculate conception" is a logically derived one. That is, pre-incarnate Jesus, knowing Mary was to be his mother, saw fit to give her the gift of perfection for His benefit and for the removal from him of "original sin," a doctrine that is itself suspect.
My greek text is also not with me. In my software the word is from karitow (vocative, 2nd person singular, past perfect participle) to favor highly, to have shown grace to [Liddell & Scott Lexicon]; to show kindness to [Louw & Nida Lexicon].