The Seed, Christ, comes from Mary, is born of woman. That is what it means by "her seed". But the baby she gives birth to is the Seed that will crush Satan's head. You cannot say in one breath that yes the Seed is Jesus and in the next say the seed of Eve is referring to all mankind. Scripture clearly states that it is in Adam, not Eve, that mankind went to war with God, not the devil (see Psalm 2). It is all in Adam who are the seed of Satan, and the seed in that case is not literal if Satan is a fallen angel. Angels have no seed which is why they do not marry. The seed of the woman is a literal "he". The seed of the serpent are the spiritual offspring of Satan. All mankind since Adam's fall.
1 John3:10 By this it is evident who are the children of God, and who are the children of the devil"" whoever does not practice righteousness is not of God, nor is the one who does not love his brother. 8. Whoever makes a practice of sinning is of the devil, for the devil has been sinning from the beginning. The reason the Son of God appeared was to destroy the works of the devil.
In verse 8 we even see that to say there is a war going on between Satan and mankind is incorrect. The war is between Christ and the devil. Between God and the serpent.
Here is another reference that illustrate my point.
Gal 3:16 Now the promises were made to Abraham (a seed bearer in the genealogy of Jesus) and to his offspring. It does not say, "And to offsprings," referring to many, but referring to one. "And to your offspring" who is Christ.
