The church has always held that Mary was sinless from conception. Show me one historian or church authority writing prior to the onslaught of protestantism that recorded that the church taught otherwise. (Even schismatic Luther held belief in the doctrine of the Immaculate conception!)
Besides, the atonement WAS applied to her prior to conception.
Mary was never sinless....the only sinless person ever or ever will be is Jesus Christ. She was the birth mother of our Savior. It pretty much stops right there as to anything regarding her divinity which she never had nor does she now.