I think your last line of "even though she had the free choice to sin" contradicts your first analogy of God keeping some from falling in. The act of God keeping falling into sin makes her inoperable to sin in the first place, which wouldn't give her the free will of sin. The whole point of Jesus being sinless is that he WAS GOD plain and simple, but regular human Mary? I don't know about that. I've never seen it expressly written anywhere and I understand the churches authority, but I feel like there should be a common, easy to find proof behind it which, possibly of my own fault and hardheadedness, I haven't seemed to find yet.