The following is, again, an excerpt from email correspondence with a Protestant in good faith.
Mary does not make up for any insufficiency in God; God alone is truly enough. However, God is our Lover, and lovers don't give merely enough. He gave us Mary as a guide to imitate in teaching us how to radically conform our will to God's holy will. A simpler point that St. Thomas Aquinas makes:
For it is a great thing in any saint that he has so much grace that it suffices for the salvation of many, but when enough is had for the salvation of all the men in the world, this is the greatest, and so it is with Christ and with the Blessed Virgin.
Perhaps this could be fleshed out in a syllogism.
Premise 1: When one holy man or woman, by their conformity of their will to God's, leads 100 people to Christ and therefore to salvation, that is an exceedingly great thing. Now, obviously that person did not technically save their souls, strictly speaking, by doing the supreme act of redemption that Our Lord did on the cross; however, if it were not for that person conforming their will to God's holy will, those people would have otherwise missed out on salvation. In that same vein, how much greater is it when a holy pope leads hundreds of thousands of souls to salvation who otherwise were on the path to destruction? Further, it is significantly greater that the conformity of one person's will to God's will led to the salvation of the entire world, although that person in and of themselves did not perform the actual act of redemption, the Sacrifice of the Cross.
Premise 2: Mary conformed her will to God's holy will in a perfect manner in her fiat. "Behold the handmaid of the Lord: be it done to me according to thy word." (Luke 1:38) This singular act of submission to the holy will of God led to the salvation of the entire world because if she would have rejected it, Our Savior would not have come through her.1
Conclusion: Therefore, Mary's conformity of her will to God's is the greatest and most honorable of any mere human.
It is exceedingly great that her sanctity brought people closer to God, or rather, God to people, more so than any other human in history, and we can see how this does not detract from the merits of Christ.
Not only did the Blessed Virgin play an instrumental part in bringing Our Lord to Earth; she never let herself overshadow Christ. She always pointed us to Our Lord. (John 2:5)
She is like the moon in comparison to the sun. The sun is bright and glorious in and of itself. The magnificence of the moon rests solely on its ability to reflect the light of the sun. When we look at a bright full moon, we become mesmerized not because of the light it emits, but because of the sun's light it reflects so magnificently.
Great saints use this analogy frequently. For example, St. Louis de Montefort takes the analogy to another dimension.
"When we see her, we see our own human nature at its purest. She is not the sun, dazzling our weak sight by the brightness of its rays. Rather, she is fair and gentle as the moon, which receives its light from the sun and softens it and adapts it to our limited perception."
Obviously, this is not to say that God can’t work around this. The extent of this point is to demonstrate that what happened in reality is that her conformance to the will of God did wind up being the bottleneck through which the salvation of the entire world did come through.
Have you ever considered that perhaps it is YOU who are the stumbling block? Just a thought.