Mary's fiat and predestination....

From a Predestinarian's point of view, Mary was predestined in Genesis.  In fact, she is predestined in the (created) stars -- Virgo!
As such, we know from St. Augustine's definition of Predestination:  It is simply the preparation of and for Grace.
That is, for each of the Elect, their Will is prepared and the Graces for those Wills are also prepared.
I.E.  Not all Wills are created  equal,  and  Not everyone receives the same amount of Grace.
The converse of this is:  Some Wills are not created for salvation and some do not receive Grace.  HARD SAYINGS INDEED.

Now, to the FIAT  of Mary.
By Mary's WILL, she said Yes.

For the Pelagian, the Will is inherently supreme.
For  the Catholic, Grace supersedes the Will.

The title of Co-Redemptrix boils down to an act of Will -- the Fiat of Mary.

Noticably, the push for the title Co-Redemptrix comes mostly in the last 500 or so years......  mainly during the times that Pelagian forces in the Church had increased  their influence in the Church.

The latest Vatican Theological  Commission, which commissions of late have pushed for more nonsense, has applied its wiles to  have Mary declared Co-Redemptrix!
And why?  The Will of Mary is the cause or co-cause of our Redemption.  If this is so, and Mary said NO, there would be no Redemption.  But since God's Grace is SOVEREIGN, No One's Will (remember -- all Wills are created by God) can divert or overcome God's plan for man.

I am  guessing that the push for the supremacy of Mary's Will in the Redemption is being authored by those Pelagian forces bent on humanizing and de-supernaturalizing and de-Incarnationalizing the Roman Catholic Church, outside of which there is neither Salvation OR the remission of sins.
Further, because of her  cooperation by saying Yes to  become the Mother of God, does that make her Co-Redeemer?

What about all those that came before in the lineage of Our Lady from the time of Adam and Noah?  Should they not be also added to the list of Co-Redeemers?  As well as the Apostles and disciples  that brought forth the Fruits of the Redemption to the masses of people  by their spread of the Faith and Baptism?
Baptism, after all, redeems the particular soul and marks  them with the seal of Redemption.  So all those  baptizers out there, laymen and laywomen and priests, bishops, etc. etc. can also be Co-Redeemers.

Thus we see, in the very same sense that  these others can be  co-redeemers, Our Lady can be  considered as such. 

However, to aver that by not claiming her to be Co-Redemptorix is to deny her co-operation in the salvific  plan of God is way over the top.