I was doing some research and even though could find different resources on devotion to Mary, Mary as the mother of God, I couldn't find anything useful about Mary as mediatrix / "to Jesus through Mary".
What did the early church fathers have to say about it?
there is no exact explicit mention of mother mary as the "mediatrix" in the first few centuries. but the root of the teaching was definitely with the church fathers.
St. Irenaeus (130-202), in his famous Against Heresies (bet. 180-199) wrote:
“. . . so also Mary . . . being obedient, was made the cause of salvation for herself and for the whole human race . . . Thus, the knot of Eve’s disobedience was loosed by the obedience of Mary. What the virgin Eve had bound in unbelief, the Virgin Mary loosed through faith.”
(3, 22, 4; from W. A. Jurgens, The Faith of the Early Fathers, Collegeville, Minnesota: Liturgical Press, 1970, vol. 1, p. 93, #224)
“. . . for in no other way can that which is tied be untied unless the very windings of the knot are gone through in reverse: so that the first joints are loosed through the second, and the second in turn free the first . . . Thus, then, the knot of the disobedience of Eve was untied through the obedience of Mary.”
(Against Heresies, III, 22,4; from William G. Most, Mary in Our Life, Garden City, New York: Doubleday Image, 1954, 25)
St. Irenaeus wrote in Against Heresies, III, 21, 7:
On this account also, Daniel, foreseeing His advent, said that a stone, cut out without hands, came into this world. For this is what “without hands” means, that His coming into this world was not by the operation of human hands, that is, of those men who are accustomed to stone-cutting; that is, Joseph taking no part with regard to it, but Mary alone co-operating with the pre-arranged plan. For this stone from the earth derives existence from both the power and the wisdom of God. Wherefore also Isaiah says: “Thus saith the Lord, Behold, I deposit in the foundations of Zion a stone, precious, elect, the chief, the corner-one, to be had in honour.” So, then, we understand that His advent in human nature was not by the will of a man, but by the will of God.
St. Ambrose of Milan (c. 339-397) wrote:
Mary was alone when the Holy Spirit came upon her and overshadowed her. She was alone when she saved the world — operata est mundi salutem – and when she conceived the redemption of all — concepit redemptionem universorum.
(in Mark I. Miravelle, ditor, Mary: Coredemptrix, Mediatrix, Advocate: Theological Foundations, Santa Barbara, Califiornia: Queenship Publishing, 1995, p. 14; from Epist. 49,2; ML 16, 1154)
She engendered redemption for humanity, she was carrying, in her womb, the remission of sins.
(in Miravelle, ibid., p. 14; from De Mysteriis III, 13; ML 16,393; De instit. Virginis 13,81; ML 16,325)
St. Ephraem of Syria (c. 306-373) called Mary the “dispensatrix of all goods.” (in William G. Most, Mary in Our Life, Garden City, New York: Doubleday Image, 1963, 48)
Basil of Seleucia (died c. 458) referred to her as the “Mediatrix of God and men.” (in Most, ibid., 48)
St. Cyril of Alexandria (d. 444) wrote:
“Hail, Mary, Mother of God, by whom all faithful souls are saved [sozetai].
(in Miravelle, ibid., p. 13; from MG 77, 992, and 1033; from the Council of Ephesus in 431)
St. Andrew of Crete (c. 660-740) referred to Mary as the “Mediatrix of the law and grace” and also stated that “she is the mediation between the sublimity of God and the abjection of the flesh.”
(Nativ. Mariæ, Serm. 1 and Serm. 4, PG 97, 808, 865; in Miravelle, ibid., 283)
St. John of Damascus (c. 675-c. 749) spoke of Mary fulfilling the “office of Mediatrix.”
(Hom. S. Mariæ in Zonam, PG 98, 377; in Miravelle, ibid., 283)
The Protestant reference Oxford Dictionary of the Christian Church (ed. F. L. Cross, 2nd ed., Oxford Univ. Press, 1983, p. 561), states concerning Patriarch Germanus:
“Mary’s incomparable purity, foreshadowing the doctrine of the Immaculate Conception, and her universal mediation in the distribution of supernatural blessings, are his two frequently recurring themes.”
William Most comments:
“Mary, says St. Irenaeus, undoes the work of Eve. Now it was not just in a remote way that Eve had been involved in original sin: she shared in the very ruinous act itself. Similarly, it would seem, Mary ought to share in the very act by which the knot is untied — that is, in Calvary itself.”
(in Most, ibid., 25)
“Just as the human race was bound over to death through a virgin, so was it saved through a virgin: the scale was balanced — a virgin’s disobedience by a virgin’s obedience.”
(Against Heresies, V, 19, 1; cited in Most, ibid., 274)
And also the well-known Protestant patristics scholar J. N. D. Kelly says:
"The real contribution of these early centuries, however, was more positively theological, and consisted in representing Mary as the antithesis of Eve and drawing out the implications of this. Justin was the pioneer, although the way he introduced the theme suggests that he was not innovating . . . Tertullian and Irenaeus were quick to develop these ideas. The latter, in particular, argued [Against Heresies, 3, 22, 4; cf. 5, 19, 1] that Eve, while still a virgin, had proved disobedient and so became the cause of death both for herself and for all mankind, but Mary, also a virgin, obeyed and became the cause of salvation both for herself and for all mankind. “Thus, as the human race was bound fast to death through a virgin, so through a virgin it was saved.” Irenaeus further hinted both at her universal motherhood and at her cooperation in Christ’s saving work, describing [Ibid, 4, 33, 1] her womb as “that pure womb which regenerates men to God.”
(Early Christian Doctrines, San Francisco: HarperCollins, revised edition of 1978, 493-494, emphases added)
More citations are in https://www.patheos.com/blogs/davearmstrong/2017/03/mary-mediatrix-church-fathers-vs-james-white.html
Among the early Fathers, Irenaeus of Lyons comes closest to calling Mary a mediatrix. Indeed, he could be said to even go beyond the designation of Mary as mediator to make her a new Eve, the giver or rebirth and a universal savior.
Just Jesus was the second Adam (1 Cor. 15:45) so, for Irenaeus, Mary was the new Eve and the rescuer/savior of those who have faith:
And if the former did disobey God, yet the latter was persuaded to be obedient to God, in order that the Virgin Mary might become the patroness (advocata) of the virgin Eve. And thus, as the human race fell into bondage to death by means of a virgin, so is it rescued by a virgin; virginal disobedience having been balanced in the opposite scale by virginal obedience. (Adv. Haer. 5:19, 1)
Irenaeus also refers to Mary's "pure womb which regenerates men unto God." (4:33,11) It seems he considers Mary as the means by which Christians are reborn.
The word "mediatrix" does not appear. But it is hard to imagine a higher Mariology. For Irenaeus, Mary is a new Eve, the savior of the human race, and also its means of regeneration.
