Was Mary's Fiat Willful?

I've been following a discussion this week over at ISCA regarding the question of whether Mary willingly consented to become the Mother of God. Usually topics over there get recycled every few years, sometimes every few months, depending on how the conversations drift and how often new blood comes in asking to rehash old subjects. The conversation is pretty mellow and charitable right now, but I think they were kind of dancing around what may be the central point. I'll summarize a few of their ideas, pro and con, and then get to what I think is the critical junction.

"Didn't God respect Joseph as Mary's husband?"
Certainly, but marriage's ultimate purpose isn't the honor and satisfaction of the husband.

"Isn't not asking for sex rape? Isn't 'Let it be done to me' the typical response of a rape victim too despairing to resist?"
First of all, "'Let it be done' as 'I give up'" is one of the most profoundly bitter interpretations of any historical or literary event I have ever seen, one that says volumes more to me about the interpreter than the event itself.

Technically, there was no sex, since Mary remained a virgin, so rape isn't any more accurate a term than adultery, but the question of God using Mary as a tool without her consent may still be worth asking. Let me broaden the question first. If Mary were willing, would it still be God objectifying her as a mere, albeit consenting, tool? This question comes up a lot more than just in regard to the Incarnation; how many times have we heard people rail against being treated as (or hearing the unique roles of womanhood being reduced to) a baby-making machine? If being a mother isn't necessarily objectifying--and it's not--then Mary's motherhood isn't, either.

"Mary didn't really have a choice. Even if a woman might consent to rape by an overpowering attacker in the interest of surviving the attack, no one could resist God's will."
This argument sounds more like an atheistic assertion--no worthwhile god would violate our free will, an omnipotent god can't help but impinge whenever it interacts with us, therefore there is no god who deserves our adoration and is capable of interacting with us--than a complaint against any particular Marian dogma. The Fiat is just one instance where we can see the Deity's sovereignty meeting but not invalidating the free will that each of us has. It's a mystery, sure, but if you can accept it elsewhere, there's no reason not to accept it here.

God is infinitely more powerful than any rapist, but He is no bully.

"'Behold I am the handmaid of the Lord' is a fancy way of saying she was oppressed as a woman and conditioned by her culture to obey authority, especially God."
We all should be so oppressed that we are conditioned to obey God. He wants what's best for us and knows better than we do, so it's not like by disobeying we're going to get something better.

"That she would not hesitate to accept the role of Theotokos is probably the key reason she was destined by God for that role."
Ah, free will and causality again. This point leads pretty much to the point I wanted to make.

Given a choice between good and evil, the righteous man sees no choice at all. He still has the will to choose other than the good, but he does not imagine that God is slighting him for providing only an appealing option and an unappealing option. Most of us make such non-choices every day, and even say "I had no choice" when an alternative to some course of action is not so much unreal as unthinkable, without even looking at it as external coercion. The Bible tells us not to kill; how often do we really need to be reminded of that commandment, and do we really feel oppressed when someone does?

Making such a choice is not always clear for us, but God, knowing Mary would accept His Motherhood, set aside Mary as special from the moment of her conception, and caused her to be Immaculately Conceived, free from the taint of sin. If a friend or loved one asks you to do him or her a favor, is your first reaction to gauge the political impact on your autonomy and the convenience of the favor, or to see how well you can help the person? How much more willing would she naturally be inclined to be towards doing the good than your average good person off the street?