I'm not at all interested in starting an argument on this, but how do you reconcile the Catholic doctrine of Mary's immaculate conception - whereby she is said to have been conceived without "original sin" and to have remained sinless throughout her entire life - with what you are saying here about Mary's freedom of will to accept or reject the opportunity God had for her? Think in terms of the possibility that, if she had said "no", what would have happened about her sinlessness? Would she have then become a "retroactive" sinner (her sin being the rejection of God's will) and been as unrighteous as all mankind, continuing to sin further and needing the redemption that is in Christ?
It sounds to me like there is a contradiction with the way you phrase your opinion of "free will" applying to Mary. Her being created without a sin nature would have made her incapable of saying "no" to God, but you crudely say that would make Mary "God's brood mare", his "rent-a-womb" and make the Holy Spirit a "rapist". It is only the logical conclusion of your OWN argument and highlights the error of pronouncing Mary as a created sinless vessel but with a free will.
Mary was chosen by God, but sometimes even those chosen by God go astray. We see that in David, Solomon, Abraham, etc. The list goes on.
Did their chosenness negate their free will in choosing to do what God told them they should do? No.
So why is Mary any different? She had the exact same choice that Eve had - she could choose to go through with it, or she could choose otherwise. But - God left it up to her. She could have chosen otherwise, and God would have left her to her graces.
This is why the Catholic church teaches that man was saved through the obedience of Mary whereas man was condemned through the disobedience of Eve. Mary chose to bear Christ.
And Mary is not a vessel - she is a woman a person. Christ is truly her son just as any other child!