Indeed. So Mary was given grace, the act was once complete, and her state now is referred to by the angel as such that resulted from the complete bestowal of grace.
in the case of Christ we do have this confirmation from other Scripture [...] We have no such confirmation with Mary.
It is, first, a wrong premise to seek every theological insight from a direct scriptural text. The Church produced other teachings beside that contained in the Scripture; mature Catholic mariology developed over time. That the Protestants wish to limit their cognitive ability to a direct instruction from canonical scripture is a part of the reason why Protestantism is a theological fantasy, whose only effect on the Christendom was to scatter the flock and lead it to loss of faith. Now, no single word in the Bible is authority for you unless it is repeated twice. Why stop at that? Perhaps we should seek a triple confirmation? Is "κεχαριτωμενη" in the context it is used, with the grammatical meaning that you describe, a word of God or not?
Nowhere in the scripture do we see Mary somehow losing that state of grace. She in fact is the only one beside St. John to stick with the Lord to the very end of His mission, at the Cross and at Ascension. Followers of Christ venerated her; the beloved disciple was adopted by her. A reasonable reader of the Holy Scripture who reads it in order to learn rather than in order to find anti-Catholic phony prooftexts sees modern mariology all in there.
So if a person does NOT believe the way the RCC spins it; they are unreasonable?
OK then...
Count me in!
No cognition can occur using a single atomic data-point occurring in a vacuum. That simply is not how thought and language occur. Even two verses might not be enough, if a third verse occurs which precludes some variant of misunderstanding the first two.
Look at your own post. How many "verses" did you deploy to convey your meaning? How long is the canon of the law and the prophets? Yet Christ said the sum of them is to love God with everything you've got, and to love your neighbor as yourself. If Scripture had simply opened with the single word "love," and that's it, Christ would have been unable to infuse that statement with any meaning. The many, many words God has given us are our friends, not our enemies, in understanding the overall message of God. Otherwise he would not have given them to us.
The RC's here frequently complain about the proliferation of non-RC denominations, yet the false teachers who rise up use precisely this mechanism, of failing to compare Scripture with Scripture, and building some novel heresy based on a purported hidden meaning buried in a single word which only they have received the gnosis to properly understand. You may be comfortable with that. I am not.
As for the perfect participle "having been favored/graced/shown kindness," your response suggests you are implying two "completes" when there is only one "complete" that is grammatically justified. The completeness of the perfect is temporal, not qualitative. It only says the event is over. It does NOT describe the ontological nature of the event. That remains for the root and the broader context to determine, which analysis often entails the use of many passages for confidence that it is correct. This is simply due diligence, aka rightly dividing the Word of God. It's a bit more work, but it's the right thing to do.