So Mary's choice was meaningless.
One cannot choose or un-choose God's will.
And I'm guessing that in the Calvinist way you intend this, 'one' means everyone; and everyone's choices are meaningless.
This, I believe would comport with my initial: You describe a cosmos where human life and decisions are meaningless. Rather than, your reply: "A desired goal of many christians is to exercise our will in conjunction with God's." which doesn't square with your last reply.
So, yes, I think your positions here are at odds with each other.
Mary believed.
Was it a choice for her to believe or did God first do the convincing by sending her an Angel?
The sun is shining today. I believe the rain is not falling not because I chose to believe it but because the sun convinced me this was so.
Mary was not presented with a choice in whether or not Jesus would be born through her. This was up to God and through the Angel He told her what was about to take place.