Mary is not relevant as she didn't mother both the offspring of God and the offspring of Satan as did Eve. Christ is the offspring of Eve, his maternal antecedent. Who do you think is/was the offspring of the devil, and who is their maternal antecedent? And how does a spirit-being produce offspring?
The prophecy contains two 'groups', the principals, Christ and Satan, and the spiritual offspring of Eve and Satan. Two groups, two prophecies. Cain and Abel began the fulfillment of the physical offspring part of the prophecy, and the enmity between God's chosen and rest of the world continued from that time forth.
God cursed Cain on Abel's behalf. How or when Cain died physically isn't important. Note that while Christ crushed the head of Satan he isn't killed, so 'crushed' has a different meaning, perhaps 'deposed' would fit as Satan's rule will be destroyed.
God was speaking to Satan, who was in control of Eve's mind. He was looking into Eve's eyes but speaking to Satan just as Jesus was looking at Peter but speaking to Satan.
God cursed Eve to have sorrow in childbirth. Most believe this means painful or difficult labor, however as were the Hebrew women in Egypt Eve would have had easy deliveries of children due to excellent health. The sorrow in both conception and childbirth was the mental anguish because of what happened to Cain and Abel, and perhaps others of her children.