Mary had to be atoned for? Certainly. Did the atonement have to be applied after conception? Not necessarily. By Whose decision and through Whose power and ability? God alone. Since it's late, I'll refer you to an explanation from Catholic Answers which I will here excerpt:
[Objectors’] chief reason for objecting to the Immaculate Conception and Mary’s consequent sinlessness is that we are told that "all have sinned" (Rom. 3:23). Besides, they say, Mary said her "spirit rejoices in God my Savior" (Luke 1:47), and only a sinner needs a Savior.
Let’s take the second citation first. Mary, too, required a Savior. Like all other descendants of Adam, she was subject to the necessity of contracting original sin. But by a special intervention of God, undertaken at the instant she was conceived, she was preserved from the stain of original sin and its consequences. She was therefore redeemed by the grace of Christ, but in a special way—by anticipation.
Consider an analogy: Suppose a man falls into a deep pit, and someone reaches down to pull him out. The man has been "saved" from the pit. Now imagine a woman walking along, and she too is about to topple into the pit, but at the very moment that she is to fall in, someone holds her back and prevents her. She too has been saved from the pit, but in an even better way: She was not simply taken out of the pit, she was prevented from getting stained by the mud in the first place. This is the illustration Christians have used for a thousand years to explain how Mary was saved by Christ. By receiving Christ’s grace at her conception, she had his grace applied to her before she was able to become mired in original sin and its stain.
I recommend this tract in it's entirety to anyone looking for a very basic explanation of the Immaculate Conception and Assumption.
Nota Bene to Doc: This is a Caucus thread, so I'll ask you to please save outside objections for an open thread. However, if I understand correctly, questions respectfully asked are fine.
"Mary had to be atoned for? Certainly."
Why certainly?
According to our doctrine she never once committed a personal sin.
And according to our doctrine of the Immaculate Conception, she was not conceived with original sin either.
I guess the argument about being grabbed from the brink of the pit before falling into it is a pretty good one, all in all.
But it does raise a follow-on question as to why God has decided to impose the requirement of "atonement" on people. He has decided to forgive, but has arbitrarily imposed a procedure for forgiveness. It's interesting. Why?
Perhaps simply because he wants to.
But this has the effect of causing a lot of people to be condemned.