A moment of clarity.

I"ve tried to come up with how to explain the idea of Mary and her sinlessness. I've tried the manhole analogy, which didn't click. [Two men walking down the street. One falls into a manhole; you get a rope and help him out. The other is on the same trajectory, but you holler "Look out!" and he avoids falling in it. Which did you save? Both.]
So I thought of another one.

Say there's this doctor. A pediatrician. He sees this patient in his office, where he recognizes the potentially fatal early signs of... diptheria, say. He immediately treats the symptoms, tells the parents to get the child to the hospital, calls an ambulance to get the child there, and is in touch with the staff at the hospital regularly. Now, the parents had just though the child mildly sick and were there for amoxicillin, but the doc knew how serious it could be and by acting saved the child's life.
The next patient is in for a well-baby check and gets the DTaP shot, thus eliminating the chance of the child getting diptheria.

Which child did the doctor save? Both, right? But they both needed the doctor, they didn't do it on their own.

That's how it is with Mary. She received the wholeness of God's grace at conception--she isn't sinless because of anything she did but what God did. She still needed God's grace; she just got it before she sinned.