First of all the verse is about some future event, it is a prophecy, so it is not logical to ask in what way Mary's heel has been bruised. It says that the woman's heel will be bruised by the serpent. The rest of my answer to this question is found in Revelation 12, (below).
As for the "it" question in the translation, there is no "it" in Hebrew. However, both 'hu', (Hebrew for 'he'), and 'he', (Hebrew for 'she'), can be translated "it" depending on the context. I realize that the translation of this verse is a controversial one to many, and not being a linguistic scholar in biblical languages I must defer to the Church's official translation in the Latin Vulgate and its English translation, the Douay Rheims. I also defer to two thousand years of Catholic tradition, to what the early Church Fathers and the Saints have written about the understanding of this verse over the millenia.
Back to the first question, it is revealed in the Apocalypse, (Revelation 12), that "the woman and her seed" are Mary and Her Son Jesus. So it is both Jesus and Mary Who will crush the head of the serpent, Mary being given the power and authority to participate in the "crushing" of the sepent by Jesus Christ. :
1 "And a great sign appeared in heaven: A woman clothed with the sun, and the moon under her feet, and on her head a crown of twelve stars: 2 And being with child, she cried travailing in birth, and was in pain to be delivered. 3 And there was seen another sign in heaven: and behold a great red dragon, having seven heads, and ten horns: and on his head seven diadems: 4 And his tail drew the third part of the stars of heaven, and cast them to the earth: and the dragon stood before the woman who was ready to be delivered; that, when she should be delivered, he might devour her son. 5 And she brought forth a man child, who was to rule all nations with an iron rod: and her son was taken up to God, and to his throne."
6 "And the woman fled into the wilderness, where she had a place prepared by God, that there they should feed her a thousand two hundred sixty days. 7 And there was a great battle in heaven, Michael and his angels fought with the dragon, and the dragon fought and his angels: 8 And they prevailed not, neither was their place found any more in heaven. 9 And that great dragon was cast out, that old serpent, who is called the devil and Satan, who seduceth the whole world; and he was cast unto the earth, and his angels were thrown down with him. ------ --"
13 "And when the dragon saw that he was cast unto the earth, he persecuted the woman, who brought forth the man child: 14 And there were given to the woman two wings of a great eagle, that she might fly into the desert unto her place, where she is nourished for a time and times, and half a time, from the face of the serpent. 15 And the serpent cast out of his mouth after the woman, water as it were a river; that he might cause her to be carried away by the river."
16 "And the earth helped the woman, and the earth opened her mouth, and swallowed up the river, which the dragon cast out of his mouth. 17 And the dragon was angry against the woman: AND WENT TO MAKE WAR WITH THE REST OF HER SEED, who keep the commandments of God, and have the testimony of Jesus Christ". ---
I don’t see where in that passage the woman does any “crushing”. She just gets help to run away, and help to be protected from the beast’s attacks.