Since we're Stephen's namesakes it's great to know he was "full of grace."
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.