We discussed this elsewhere. Being "full of grace" (which as you know is not the Greek phrase) or "highly favored" (which is much closer to the formal, "mundane" Greek) provides no evidence whatsoever of the uniquely RC doctrine of immaculate conception. In fact, in the critical text Greek, Stephen is said to be full of grace (Acts 6:8), and in Ephesians 1:6, all believers have this same exact grace, which in Ephesians is rendered as "accepted" in the Beloved. So if neither Stephen nor believers in general are categorically sinless, despite being up to their ears in grace, the Lukan passage cannot imply that about Mary either.
πληρης πιστεως και δυναμεως
Why St. Jerome translated "grace" instead of "faith" I don't know.
The only other use of "κεχαριτωμενη" is in Ecclesiasticus (Sirach) 18:17
| Ecclesiasticus (Sirach) | |||
| English: Douay-Rheims | Greek OT: LXX [A] Unaccented | English: King James Version | |
| Ecclesiasticus (Sirach) 18 |
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| 17. | Lo, is not a word better than a gift? but both are with a justified man. | ουκ ιδου λογος υπερ δομα αγαθον και αμφοτερα παρα ανδρι κεχαριτωμενω | Lo, is not a word better than a gift? but both are with a gracious man. |
KJV again is working hard to obscure the Bible: Mary is "highly favored" but the idealized "son" in Sirach is "gracious", -- same word and similar context. Mary is described as "full of grace" in Luke and the idealized son as "justified" in Sirach, and indeed he is to be "justified" in anticipation of "the wrath that shall be at the last day" (Ibid. 18:24). The juxtaposition of these two occurrences of κεχαριτωμενος explain the intended meaning of the Archangel's address to Mary: she, while a young girl, is already like the saints in heaven, the justified.