You have a very good point, and I commend you for that. However, we do also need to consider that Heaven is what many have called, "The Upside-down Kingdom," and thereby isn't like human kingdoms. The last is first, and the first is last. The greatest feats of God's power are done through the least of us humans, and the King Himself desires intimacy with us, rather than having someone running in between Him and us.
Another thing is that God, existing outside of time, knows everything that will happen before it does, and therefore cannot be influenced, at least not in the sense that humans are. There are, yes, instances where it appears that God acts in accordance with human wills, but it is more comparable to a human father, who, already knowing their little one will need help, nonetheless waits until they ask for it.
None of us are good enough for God. We are all sinners, fallen short of God's glory, and so have to be freely justified by His grace that came through Christ Jesus. Mary was a human like the rest of us. No one really knows why God chose her; all it says is that she was highly favored. Certainly we see throughout her story she displays a very Godly and wise character, moreso than most modern girls in her age range (14-ish, according to biblical scholars), but she wasn't perfect. However, God used her nonetheless, and that is what matters.
I apologize if I accidentally took on an agressive tone. I find it difficult to make a point without sounding standoffish.
And, once again, thank you for responding. I understand that you have a lot of things on your plate, and so it means a lot to me that you bother to answer my questions.