According to the Catholic commentators, was Mary the original candidate for mother of God? Is it possible to think that another woman before Mary had been asked by an angel to be the mother of God, but the incarnation did not happen at that time because she rejected the proposal?

In the other words, does the Catholic Church teach that Mary the first woman asked to be mother of God?
In Catholic teaching and theology, there is what is called a vocation1, i.e. a calling. From all eternity, God has called each one of us to be what he wanted each one of us to be. This calling is unique and unrepeatable precisely because each person is unique and unrepeatable2.

1. We define personal vocation as God's call and plan for one's entire life. cf. GERMAIN GRISEZ ON PERSONAL VOCATIONS | EWTN.
2. cf. b. Unique and unrepeatable | B. OPENNESS TO TRANSCENDENCE AND UNIQUENESS OF THE PERSON in COMPENDIUM OF THE SOCIAL DOCTRINE OF THE CHURCH.

The following sample biblical passages are offered in support:


  Matt 20:23
  (RSVCE)
  23 He said to them, “You will drink my cup, but to sit at my right
  hand and at my left is not mine to grant, but it is for those for whom
  it has been prepared by my Father.”


And 


  Jn 15:16
  (RSVCE)
  16 You did not choose me, but I chose you and appointed you that you
  should go and bear fruit and that your fruit should abide; so that
  whatever you ask the Father in my name, he may give it to you.


And


  Is 43:1
  (RSVCE)
  43 But now thus says the Lord, he who created you, O Jacob, he who
  formed you, O Israel: “Fear not, for I have redeemed you; I have
  called you by name, you are mine.


Thus Mary was God's choice to be his Mother and now is the only choice because she said yes! And we thank God and her for that.

What could God have done if Mary had said no?

With God all things are possible3. In Catholic teaching and theology there are infinite number of ways by which God could have accomplished the redemption. We thank the LORD and his Mother for having agreed to be the New Adam and Eve.

3. cf. Matt 19:26



Please see: CCC 490ff.



Postscript

What was the calling of Eve, what was her glory? 

She was the first woman, the mother of all living, among her children, Mary the Mother of God, and her Son, the Son of man4.

I am sure Eve is not complaining.

4. cf. Matt 8:20.