The argument that “full of grace” is used identically of both Mary and Stephen is not supported in the Greek.
That's exactly right.
This is because the Catholics are depending on the Latin Vulgate translation of the Bible done by St. Jerome (4th Century.) Here, you find "ave gratia plena" translated "Hail full of grace.'"
We know that the New Testament was written in Greek--not Latin--but the Roman Catholic Church has derived its doctrine from the Latin translation--not the original Greek. They are using a false translation. Now they're stuck- since so much is invested in their worship, adoration, and prayer ("Hail Mary") to Mary. They would lose credibility if they corrected the error.
Unfortunately, this means that millions of Catholics will continue to look to Mary for help--not Christ who is truly full of grace.
Of Mary, the Greek word [Lk 1:28] is kexaritomena (highly favored), of Christ (and Stephen) the Greek phrase [Jn 1:14 and Acts 6:8] is plaras karitos (full of grace).
You are correct ... the Acts passage actually explicitly charges that Stephen was full of grace (πλήρης χάριτος - Acts 6:8)
Whereas ... Luke 1:28 the word is κεχαριτωμένη - a perfect passive participle of χαριτόω meaning to bestow favor on, favor highly, or bless. With the force of the imperative used as a greeting ... it simply means "Greetings, favored one ... the Lord is with you"
The Acts passage is the much clearer of the two instances.
So I assume this was what you were going to argue right?