“Mary is also an obvious exception to the ‘all have sinned’ line”
It may be so that she was without sin, but there is nothing at all obvious about it.
All we need do is read the story of the Annunciation and allow ourselves to understand the plain words of Gabriel, emissary of God.
He gave Mary the title "Full of Grace". Chaire, Kecharitomene
Here is clear scripture with a clear meaning. Mary was Full of Grace. No sin at all.
It's meaning was certainly obvious to the early Greek Fathers , who were greeks of a culture basically identical to that of St Luke's. They certainly understood the same by Chaire, Kecharitomene as we do today.
For instance Gregory Thaumaturgus (205-270 AD): wrote the following exposition on the meaning of Luke's gospel and Chaire, Kecharitomene.
O purest one
O purest virgin
where the Holy Spirit is, there are all things readily ordered.
Where divine grace is present
the soil that, all untilled, bears bounteous fruit
in the life of the flesh, was in possession of the incorruptible citizenship,
and walked as such in all manner of virtues, and lived a life more excellent than man's common standard
thou hast put on the vesture of purity
has selected thee as the holy one and the wholly fair;
and through thy holy, and chaste, and pure, and undefiled womb
since of all the race of man thou art by birth the holy one,
and the more honourable, and the purer, and the more pious than any other:
and thou hast a mind whiter than the snow, and a body made purer than any gold
It was obvious to greeks in 250 AD. They hadn't fallen into our modern habit of taking tidbits of scripture and running them like insane little DOS commands, oblivious to their actual meaning - while ignoring the plain meaning of the Angel of God.
Hope this is helpful.