(1) Kecharitomene means completely, perfectly, enduringly endowed with grace.
“completely, perfectly” are your additions to the meaning, ‘highly favored, to imbue with grace’, etc. the use of the term “grace” is thus equivalent to “favor” and has nothing whatsoever to do with sin or the degree of sin of the person favored.
(2) “Of course, Catholics agree that Mary has received grace. This is assumed in the doctrine of the Immaculate Conception: it was a grace from God which could not possibly have had anything to do with Mary’s personal merit, since it was granted by God at the moment of her conception, to preserve her from original sin (as appropriate for the one who would bear God Incarnate in her very body).”
You're citing a doctrinal assumption as support of that same doctrinal assumption, i.e., ‘the sky is blue since the the sky is blue’.
(3) The work of grace in overcoming sin displays its power (Rom. 5:20-21).
True but as Paul said this applied to all Christians that “died” in the old personality not just Mary. As Romans 5:12 says, ‘death spread to ALL men because they ALL had sinned.’ “All” leaves no exception for Mary.
(4) “The only way out of the logic would be to deny one of the two premises, and hold either that grace does not save or that grace is not that power which enables one to be sinless and holy. It is highly unlikely that any Evangelical Protestant would take such a position, so the argument is a very strong one, because it proceeds upon their own premises.”
Whether such proceeds from any Protestant premise is not the question, but whether it proceeds from Scripture. John said what cleanses from ALL sin was the blood of Christ Jesus. (1 John 1:7).
Further, in verse 10 John states that anyone who said they have not sinned was a liar.
Thus Luke 1:28 speaks to Mary being highly favored with bearing Jesus and is not concerned with sin at all.
“One possible quibble might be about when God applied this grace to Mary. We know (from Luke 1:28) that she had it as a young woman, at the Annunciation. Catholics believe that God gave her the grace at her conception so that she might avoid the original sin that she otherwise would have inherited, being human. Therefore, by God’s preventive grace, she was saved from falling into the pit of sin, rather than rescued after she had fallen in.”
You may believe as you will but nowhere is Mary said to be free from inherited sin or the subject of some invention called “preventive grace”.
I was thinking more of Homer Simpson.