I think it’s important to remind everyone of the weight of the various Vatican teaching documents… in descending order of formal authority: apostolic/dogmatic constitution, encyclical letter, encyclical epistle, apostolic exhortation, apostolic letter, letter and message. Therefore, Lumen Gentium, am Apostolic Constitution would hold a higher place than the Apostolic Letters and Encyclicals typically mentioned to support claims about certain Marian privileges
At Vatican II. Lumen Gentium very purposefully stopped short of declaring Mary the Mediatrix of *All **Graces when it said *“Therefore the Blessed Virgin is invoked by the Church under the titles of Advocate, Auxiliatrix, Adjutrix, and Mediatrix. These, however, are to be so understood that they neither take away from nor add to the dignity and efficacy of Christ the one Mediator.”
This fact becomes quite clear when you understand how the teaching was formed at the Council. Archbiship Corrado Mingo of Monreal, Italy argued at the Council that the title “Mediatrix” should be amplified to “Mediatrix of All Graces”, yet it was omitted from Lumen Gentium. Several other Council fathers made that argument as well, still it was not included. It’s a well documented fact that it was considered a compromise between disparate groups at the Council to include the title ‘Mediatrix’, let alone ‘of All Graces’.
Also at the Council, Bishop Ancel, of Lyons, France said that title “Mediatrix” was given in the Constitution, but at the same time it was given no endorsement, thus leaving the door open for further study as the topic is still heavily debated among theologians. "Perhaps the title ‘Mediatrix might be listed with the other titles, in order to avoid the impression that it is a privileged one’ he said.
To reinforce the point that the doctrine was not fully formed in the Constitution…during the Council, Cardinal Alfrink of the Netherlands said “the title ‘Mediatrix’ should not be insisted upon, since it generates such great difficulties.” There was a clear divisions between the Council Fathers on whether or not to even include the title 'Mediatrix", let alone define it… And there is no mention of ‘Mediatrix of All Graces.’
Last point, which I brought up earlier… the last formal action on this subject was in the mid-90’s, at the request of the Holy See a commission was formed to determine whether or not a 5th Marian Dogma should be defined addressing the titles Coredemptrix, Mediatrix and Advocate. The commission unanimously decided not to define a Marian dogma on those titles.