Can dogma ever be modified, and how does this relate to papal infallibility?

Dogmas are definitions for the laity, not to be interpreted, and they are directly provided by the Holy Spirit. Logically speaking, if you needed someone to interpret the Dogma for you, then the Pope would not have the final word, the final word would be left with whoever was appointed to interpret the Dogmas, and only the Pope is guided by the Holy Spirit, so that line of thinking makes no sense, and it has been condemned.

Pope St. Pius X, Lamentabile, The Errors of the Modernists, July 3, 1907, #22: “The dogmas which the Church professes as revealed are not truths fallen from heaven, but they are a kind of interpretation of religious facts, which the human mind by a laborious effort prepared for itself.”‐ Condemned

Pope St. Pius X, Lamentabile, The Errors of the Modernists, July 3, 1907, #54: “The dogmas, the sacraments, the hierarchy, as far as pertains both to the notion and to the reality, are nothing but interpretations and the evolution of Christian intelligence, which have increased and perfected the little germ latent in the Gospel.”‐ Condemned

The actions of the post Vatican II papal claimants undermine this following Dogma:

Pope Eugene IV, Council of Florence, “Cantate Domino,” 1441,ex cathedra:
“The Holy Roman Church firmly believes, professes and preaches that all those who are outside the Catholic Church, not only pagans but also Jews or heretics and schismatics, cannot share in eternal life and will go into the everlasting fire which was prepared for the devil and his angels, unless they are joined to the Church before the end of their lives; that the unity of this ecclesiastical body is of such importance that only those who abide in it do the Church’s sacraments contribute to salvation and do fasts, alms giving and other works of piety and practices of the Christian militia productive of eternal rewards; and that nobody can be saved, no matter how much he has given away in alms and even if he has shed blood in the name of Christ, unless he has persevered in the bosom and unity of the Catholic Church

Almost no one claiming to be Catholic currently believes this Dogma.

Antipope John XXIII (the second one) made peace with the so called Eastern Orthodox, John Paul II made peace with the Lutherans and the protestants signing "Joint Declarations of the Faith" he prayed with the Dalai Lama making peace with the Buddhists. He didn't hold the Assisi Prayer meeting to tell them all to convert to Catholicism, he just prayed with them and said they were all great and every religion is great, and the Catholic laity got the memo. They didn't even notice when Benedict XVI praised Martin Luther, and referred to Gaudium Et Spes as a "Counter Syllabus" to Pius IX's "Syllabus of Errors"

Benedict XVI, Principles of Catholic Theology, 1982, p. 381: "If it is desirable to offer a diagnosis of the text [of the Vatican II document, Gaudium et Spes] as a whole, we might say that (in conjunction with the texts on religious liberty and world religions) it is a revision of the Syllabus of Pius IX, a kind of counter syllabus...

All attempts to convert the Jews to Catholicism have been completely abandoned stated openly by the USCCB, this decision is lauded by the Anti Defamation League, in fact, all attempts to convert anyone have been completely abandoned by the hierarchy, even though some of laity haven't figured that out yet.

Vatican II is a new religion and a complete departure from Catholicism.

The scandals and heresies of John 23.

Teachings on Popes and Heresy

The Catholic Encyclopedia, “Heresy,” 1914, Vol. 7, p. 261: “The pope himself, if notoriously guilty of heresy, would cease to be pope because he would cease to be a member of the Church.

St. Robert Bellarmine, Cardinal and Doctor of the Church, De Romano Pontifice, II, 30: "A pope who is a manifest heretic automatically (per se) ceases to be pope and head, just as he ceases automatically to be a Christian and a member of the Church. Wherefore, he can be judged and punished by the Church. This is the teaching of all the ancient Fathers who teach that manifest heretics immediately lose all jurisdiction." St. Robert Bellarmine, De Romano Pontifice, II, 30: "This principle is most certain. The non-Christian cannot in any way be Pope, as Cajetan himself admits (ib. c. 26). The reason for this is that he cannot be head of what he is not a member; now he who is not a Christian is not a member of the Church, and a manifest heretic is not a Christian, as is clearly taught by St. Cyprian (lib. 4, epist. 2), St. Athanasius (Scr. 2 cont. Arian.), St. Augustine (lib. De great. Christ. Cap. 20), St. Jerome (contra Lucifer.) and others; therefore the manifest heretic cannot be Pope." St. Francis De Sales (17th century), Doctor of the Church, The Catholic Controversy, pp. 305-306: "Now when he [the Pope] is explicitly a heretic, he falls ipso facto from his dignity and out of the Church..." St. Antoninus (1459): "In the case in which the pope would become a heretic, he would find himself, by that fact alone and without any other sentence, separated from the Church. A head separated from a body cannot, as long as it remains separated, be head of the same body from which it was cut off. A pope who would be separated from the Church by heresy, therefore, would by that very fact itself cease to be head of the Church. He could not be a heretic and remain pope, because, since he is outside of the Church, he cannot possess the keys of the Church." (Summa Theologica, cited in Actes de Vatican I. V. Frond pub.)

Pope Pius XII, Mystici Corporis Christi (# 23), June 29, 1943: 
“For not every sin, however grave it may be, is such as of its own nature to sever a man 
from the Body of the Church, as does schism or heresy or apostasy.

When the Pope speaks with infallibility on dogma, it is known as being in Ex Cathedra.

The conclusion from the Catholic Encyclopedia is that "The broad fact, therefore, remains certain that no ex cathedra definition of any pope has ever been shown to be erroneous."

The Catholic Encyclopedia, linked above, has a very lengthy article about Papal Infallibility, including many claims of evidence countering the claim.

It must be noted that the Pope's opinions are not infallible, that his addresses to the public are not infallible, that his Homilies are not infallible, and that infallibility does not extend to the Pope's personal morals or actions.

Further information from the Catholic Encyclopedia (sorry for the long Copy/Paste):

It is only in connection with doctrinal authority as such that, practically speaking, this question of infallibility arises; that is to say, when we speak of the Church's infallibility we mean, at least primarily and principally, what is sometimes called active as distinguished from passive infallibility. We mean in other words that the Church is infallible in her objective definitive teaching regarding faith and morals, not that believers are infallible in their subjective interpretation of her teaching. This is obvious in the case of individuals, any one of whom may err in his understanding of the Church's teaching; nor is the general or even unanimous consent of the faithful in believing a distinct and independent organ of infallibility. Such consent indeed, when it can be verified as apart, is of the highest value as a proof of what has been, or may be, defined by the teaching authority, but, except in so far as it is thus the subjective counterpart and complement of objective authoritative teaching, it cannot be said to possess an absolutely decisive dogmatic value. It will be best therefore to confine our attention to active infallibility as such, as by so doing we shall avoid the confusion which is the sole basis of many of the objections that are most persistently and most plausibly urged against the doctrine of ecclesiastical infallibility.

Infallibility must be carefully distinguished both from Inspiration and from Revelation.

Inspiration signifies a special positive Divine influence and assistance by reason of which the human agent is not merely preserved from liability to error but is so guided and controlled that what he says or writes is truly the word of God, that God Himself is the principal author of the inspired utterance; but infallibility merely implies exemption from liability to error. God is not the author of a merely infallible, as He is of an inspired, utterance; the former remains a merely human document.

Revelation, on the other hand, means the making known by God, supernaturally of some truth hitherto unknown, or at least not vouched for by Divine authority; whereas infallibility is concerned with the interpretation and effective safeguarding of truths already revealed. Hence when we say, for example, that some doctrine defined by the pope or by an ecumenical council is infallible, we mean merely that its inerrancy is Divinely guaranteed according to the terms of Christ's promise to His Church, not that either the pope or the Fathers of the Council are inspired as were the writers of the Bible or that any new revelation is embodied in their teaching.