Hi, nrse; welcome to the forum!
I wondered about this, too (and actually started a thread on this when I was new to the forum). Basically, there's a difference between existential sin (ancestral and simply fallen) and moral transgression of the law. One can avoid the latter but still need repair from the former. Mary needed a savior because she was not immaculately conceived, but the Church does teach that, at the least, she was the least sinful human ever to live and, at most, without personal sin but in need of cleansing of ancestral fallenness (many Orthodox authors cite the Annunciation, rather than her conception, as the moment where this happened).
Really, this is all theological opinion; the dogmatizing of this is not something we've done.
Regarding the Law, two things:
You might want to think of St. Paul or the Prophet Daniel, who were called "blameless" regarding the law, but not without having transgressed at some point.
More importantly, though, the understanding of the Church is not that our primary problem is a legal one with regards to our standing with God, but one of death reigning in our members that the Life of Christ, through our sacramental union with Him, will obliterate, and thus we are healed of a sickness, rather than acquitted of a legal failure.
Hope this helps; again, enjoy the forum!