You are right. This is a perfect example of an attempt to "harmonize" the manuscripts to fit the doctrine.
I think it is the detested "Protestants" that cling more to the Scriptures as written, and don't create their own "tall tales" or "theories" to advance their own wistful legends.
That's simply not true, bb. For example NIV is the worst of all Protestant Bibles when it comes to changing what the Greek text says.
The koine Greek term "full of grace" is πλήρης χάριτος (John 1:14, Act 6:8, older variants). In comparison, Luke 1:28 says κεχαριτωμένη which, because the grammatical form is intensive some translate as full of grace(DR) or highly favored (KJV).
The Slavonic translation, the one that most closely correpsonds to the oriignal Greek in all aspects, uses the word is благодатная or graced, and not full of grace ( преблагодатная)
So, there is no doubt that transitional flexibility, combined with doctrinal harmonizing is responsible for these variations, which ultimately lead to different interpretations, which then result in different theologies, and endless FR debates over what the Bible really says.
Even if the scripture was originally the word of God, it has long ceased to be that due to various human interventions. The only way the faith may have been preserved (by some miracle) would be through the Church Sacred Tradition, in spate of trends and translations and corruptions, and never by merely reading the Bible.
Correct, a backtranslation of "gratia plena" gives πλήρης χάριτος; that is because "κεχαριτωμένη" is a near-neologism by Luke. You will find it also once in a deuterocanonical book, forget which, but not in any of the New Testament.
The big fraud here is not the exact way to form an adjective out of grace, but the replacement of grace with favor. While occasionally it is possible -- grace is a kind of favor -- in the contect of the Annunciation it makes it sound a bit pedestrian. "Do me a favor, pal...".