I wanted to adress this very issue so that many of you here on this forum can understand that the doctrine of Mary as Mediatrix of All Graces, Advocate, and Co- Redemptrix are already infallible by the virtue of the Church’s ordinary magisterium. Let me explain…
I am sure all of you know that the Church teaches (De Fide) that at the death of the last apostle all public revelation stopped. After that no new dogmas are introduced. At times certain doctrines are “talked about” and “debated” (especially in the early Church). Often times the Popes or councils (approved by the current Pope) will teach the doctrine in an ordinary way. This occurs when the Pope teaches the doctrine, but does not solemnly defined it. The Pope may teach the doctrine in an encyclical, Apostolic Letter, Papal Bull, etc. This is an act of the ordinary magisterium. It flows logically that if a doctrine is infallible it does not all of a sudden become infallible when it is solemnly defined. It must have been infallible from at least the time of the death of the last aopstle (this is the basis of Catholicism). The Church teaches (and it flows logically) that when a teaching (on faith/morals) is taught over a constant perioid of time the teaching is infallible.9this is the ordinary magisterium) At times, this may be difficult to determine, so sometimes the Church needs to exercise her extraordinary magisterium. This means the Pope or Ecumenical council(approved by the Pope) can define the particular dogma. These type of teachings are infallible in and of themselves(obviously the doctrine couldnt be defined if it wasnt taught by the Fathers and the magisterium in an ordinary way). However, the doctrine (when it is solemnly defined) didnt/doesnt become infallible at that moment when it was defined. It was/is already infallible. The extraordinary magisterium removes all doubt and totally clarifies the issue.
Two examples of this are the Immaculate Conception and Assumption of the Blessed Virgin Mary. (solemnly defined 1854 Pius IX 1950 Pius XII) Both doctrines, obvioulsy were infallible before 1854, and 1950. Both doctrines were debated and talked about in the early Church (included in prayers of the Liturgy) and taught by various Popes and Councils (in an ordinary way). Example, the Council of Trent, 300 years before 1854, stated that Mary is to be excluded from its Decree on Original sin. After much debate and much teaching the two doctrines were solemnly defined. The Church never ever taught anything contrary to these two dogmas even though there were some who questioned or refused to accept them.
This is the same situation with the Co-Redemptrix and Mediatrix of All Graces. It was taught by a number of Fathers and taught by a host of Popes. Most recently Pope John Paul II, Leo XIII, Pius XII, Benedict XV. The Church has never taught anything denying the Mediatrix of All Graces or Co- Redemptrix.
Pope Leo XIII in Lucunda Semper(Which is totally in line with the Fathers) “Every grace is given to man from the Father to the Son through the Blessed Virgin Mary.” (quoting St. Bernard)
Benedict XV states (Inter Sodalicia 1918) "The fact that she was with her son crucified and dying, was in accord with the divine plan. To such extent did she (Mary) suffer and almost die with her suffering and dying Son; to such extent did she further surrender her maternal rights over her Son for man’s salvation, and immolated Him-insofar as she could-in order to appease the justice of God, that we may rightly say she redeemed the human race together with Christ."
These are just two recent Popes who taught this—There are a number of fathers, saints and Popes who have also taught this–Piux X, Pius XII, John Paul II—all this and not one denial from any Pope----I assert that this doctrine, even though the extraordinary magisterium hasnt acted, is infallible by virtue of the ordinary magisterium. It has been taught over a constant period of time.
Therefore, one can argue that it isnt prudent to solemnly define the dogam, but one may ot deny it----for it is infallible.