The thing is, this does not affect our salvation. There isn't anything more interesting than discussing religion, but at the end it is Jesus and His death on the cross that atoned for our sins. So I try to steer away from discussions like the one in this thread, but sometimes I am too weak and feel like I have to make my voice heard.
I think I am going to put a sign over my monitor saying looking unto Jesus, the author and finisher of our faith (Heb. 12:2) That's all that really matters.
And did it ever occur to you that God being timeless, was able to choose Mary back in the times you cite. She was indeed, chosen and preserved from Original Sin at that point in time. It was in God’s hands THEN.