Mary was FULL of Grace, and that enabled her to live a sinless life. If a blind man falls into a pit that he could not see, one could save him after the fact with a rope or ladder; but he could have been saved before the fact by being told of the pit in front of him. That is what the total INFUSION OF GRACE did for Mary; the Angel of the Annunciation even addressed her thus.
I think you mean to address your post 21 to lasereye.
With me, you’re preaching to the choir.
FWIW—credit where credit is due—Bl. Duns Scotus is the first one to explain the thing coherently. You are following in his footsteps.
For it is by grace you have been saved, through faith—and this is not from yourselves, it is the gift of God not by works, so that no one can boast.
More generally, Grace is something given to someone that they didn't earn. Mary being chosen to give birth to Jesus is an example. But it is not something that is or can be "infused". Saying that is utterly nonsensical.
Anyway, the doctrine of the Immaculate Conception is that Mary was free of original sin from the moment of her conception. As I said:
The reason Jesus had to be born of a virgin is because he would have inherited Adam’s sin if he had a biological father. Somehow Mary was able to be without the taint of original sin with a biological father but Jesus was not if we believe Catholic theology.
This "infusion of grace" thing has nothing to do with that point.
How was Mary able to be able to be free of original sin even though she had a biological father? Why did Jesus have to be born of a virgin?