Mary is NOT referenced in Genesis 3:15. The "she" that is being referenced in every instance is Eve and her offspring. Trying to inject Mary into those verses is simply a Catholic construct in an attempt to bolster a false teaching. The "big picture" show Catholicism to be a false religion and another gospel.
Doesn't that become clearer when you read in Revelation 12 that the Serpent is still at war with the Woman and the son she bore? And isn't the Woman identified as the mother of the Messiah?
Isn't she portrayed as the mother of many other offspring, namely "those who keep Gods commandments and bear witness to Jesus"?
This is one of the grandest examples of the epic coherence and continuity of Sacred Scripture from beginning to end. The continuity of Scripture from Genesis to Revelation strikes me as astonishing, an aspect of the Genius of God, miraculous.
Is Mary an offspring of Eve? Is Christ an offspring of Eve? Are you an offspring of Eve?