MD - you get into the deep theological stuff, how do you see this conundrum?
First, I;m not exactly allergic but I'm hypersensitive to "hypothesis contrary to fact." Aslan repeatedly says we are not to know what might have been. Partially because if you move one marker on the board, the other ones don't stay where they were, IMHO. So I try to recast this sort of question in positive terms. Does sinlessness compromise free will?
I think it's good to spekkerlate about sinlessness -- especially about what it does not mean. For example, we know that it doesn't mean freedom from temptation. So presumably there could still be some kind of struggle involved in Mary's assent.
But the bigger problem is I REALLY think (wait, let me get my helmet and flack jacket) that a lot of people, many of the Protestant, don't understand freedom of will properly.
(Where 'properly' is defined as "my way".)
Can God do whatever he wants? Someone got mad at me because I insisted (With Scripture to back me up: Our God is in Heaven, whatever he wills to do He does) that He can.
But that Does not mean God will do anything whatsoever.
The question pertains to the existence of evil (strictly speaking, evil does not exist) and the "ability" (if any) involved in Sinning.
I answer that sin is not an ability, a power. It is a disability, a failure of will. From our point of view because the empty vortex of evil still, um, sucks at us, evil is experienced as having power, as being an 'existent.' But from God's point of view (I know, I wrote the position paper) there is nothing there, and Satan and his power are so close to non-existent that it's hard to describe.
Another reason evil, or the ability to sin, feel like real options is that we're ignorant. Yes, I know it's hard to believe. Even Moi! Even the bestest Bible scholar and whatnot has this knobby situation in front of him, and doesn't right away know what he SHOULD do, or even what he wants/desires to do.
And ignorance is the best example, because few of us feel empowered or liberated by it. We may SAY ignorance is bliss, but we don't really act that way. Well, I mean, other than Quix.
Kidding! LUB!
(Sorry, the coffee is clambering up to the cerebral cortex here, my ego functions are not entirely on line.)
This is what came to me as I wrestled with my own Calvinism (at the time, that is) and the problem of the freedom of the blessed.
The one who sits at a fork in the road and who knows that one say leads to destruction and the other to eternal bliss, and yet finds it difficult to decide which way to go, cannot be said to enjoy the fullness of freedom.
Freedom, I say, would be to KNOW which fork to take and to have the ability to CHOOSE the correct fork (knowing it was correct) and to act on that choice.
So Mary was, in some sense, free to say to Gabriel, "Find yourself another girl," but if we stipulate arguendo that she was immaculately conceived, we would see that inexplicably to make that choice would be to lose freedom, indeed to reject it.
I idly spekkerlate that what might take me too long to realize -- better to be stoned to death for saying yes to God, than to live in some ease while saying no -- would be no-brainer for her. I might in sheer terror at the thought of having to go up to folks and say, "No, really, there was an angel; the baby is God's," forget where the promise of freedom lay. WE can imagine that, while she may have felt fear, one aspect of sinlessness would be that she was, so to speak, in better order inside, and less dominated by emotion -- which to me means MORE free than us, not less.
As to the possibility of her turning down the proposal, of course, I dunno. But Adam and Eve were subject to failure of will.... Let me stop there and get your response.
It is no wonder that God sent the Angel to Mary to say "you will" rather than "will you?"