It takes two to conceive, no? Are you asserting that Christ took no flesh from His mother?
So, even if God said that Mary was redeemed before the Cross as you say, that wouldn’t mean that Mary never sinned.
Hmm. I get your thinking, and I should have been more careful in my language. The work of the Cross involves two things: justification and sanctification. I, and most Catholics actually, consider the two of them together as the work of Redemption.
So Christ not only removes the penalty of sin (justification), he gradually makes us holy (sanctification/theosis). I am applying both of these to Mary when I am talking about the merits of Calvary. If she was perfected in grace, then she was both justified and sanctified, which meant she could not sin again, just as we will not sin again in heaven.
I’m off to Mass in a bit, so I’ll try to resume the discussion later.