Bearing in mind what you said:
1. This is like asking whether physics is retroactive. Sub-atomic physics does not alter the particles, and actions, that it describes; it is a very recent development in the sciences, which describes what was previously not understood more clearly. It does not change what it describes.
Papal infallibility does not retcon old facts of theology - it is a comparatively new formulation, that describes old facts with greater precision than hitherto, and with magisterial authority. Past definitions of doctrine, AKA dogmas, are infallibly true, whether enunciated by the Pope alone, by the Pope with the bishops of the Catholic world, or by Pope and bishops in an Ecumenical Council.
The dogma is a “latecomer”, but only in being dogmatised. All Catholic teaching outside the Bible could be called a latecomer, the Christological dogmas no less truly than the dogmas defined by Vatican I. St Paul’s teaching is a “latecomer”, compared to that of Christ. That a dogma is late in being discerned to be true, and to be a dogma, and to be definable, does not in the slightest affect its truth, its authority, its claim to be believed, or its character as truth revealed by God.
A dogmatic definition works like a microscope: it takes what was in any case part of the Apostolic Deposit of Faith, and magnifies it to make it more clearly seen than it was before, so that the Church can, by seeing it more clearly, see Christ more clearly. It is not an invention of what did not exist before. The difference in the Faith of Catholics before the definition of dogma X, and afterwards, is not that something has been tacked on from outside the Faith, but, that something that was in the Faith but not clearly seen to be in it, is at least seen clearly to be part of it. There may be many dogmas yet to be defined, that no-one now living is aware of. If defined, they will be part of the Divinely Revealed Apostolic Deposit of the Faith, and will be part of it now. The Church requires the acceptance of all that she teaches, whether made explicit already, or yet to be made explicit. The Faith is the same for all times and places - what is not the same, is the clarity of sight it is seen with.
2. In 1165, the anti-pope Lucius III beatified Charlemagne. Although this action had no authority, the Church has ratified it, and so, retro-actively made good its lack of legitimacy. It is legitimate because the legitimate and competent authoritied in the CC have legitimised it - its legitimacy does not derive from the action of the anti-pope.
There have been 40 or so anti-popes since St Hippolytus of Rome, up to Pius XIII. Very few of them have any claim to be legitimate Popes. The Great Schism of the West from 1378 to 1417 is exceptional. To cut a long story short, the competing lines of Popes are regarded as “Popes in their obediences”; by a legal fiction, the acts of the antipopes are regarded as legitimate Papal acts, as though they had been performed by the rightful, legitimate Pope, unless there is some reason to treat them as invalid actions.
3. All Popes are infallible, past and present, when they choose to exercise the infallibility of the authority to teach that is vested in their office. For:
1. Not all acts of the man who is Pope are performed in his capacity as Pope
2. Not all acts he performs as Pope are acts of teaching.
3. Not all teaching acts he performs as Pope are exercises of the infallibility vested in his office.
4. Those acts of teaching by the Pope are infallible, that are:
a. taught in discharge of his office as pastor and teacher of all Christians;
b. definitions of doctrine;
c. regarding faith or morals;
d. to be held by the universal Church.
And “these definitions of the Roman Pontiff are, of themselves, and not from the consent of the Church, irreformable”. That is, they do not have to be confirmed by being accepted by the Church; or, in technical language, they do not need to be received by the Church, to have force as infallible.
All Papal teaching - all the Church’s teaching - is
authoritative: it comes to Catholics with a claim to our interior assent, our obedience, and our submission. Regardless of whether it is infallible or not. To say “Teaching X is not taught infallibly, so I am entitled to deny it”, is a widespread but mistaken attitude. The
authentic teaching of the Church, is that which she recognises as her own teaching,
whether infallible or not - it is distinguished from the teaching of theologians and others, however pious and orthodox and edifying, that is not recognised by the Church as her own, but is not opposed to her teaching.
If these points are borne in mind, a world of confusion will be avoided.
4. The two principal sources for the dogma of Papal infallibility are:
a. The Dogmatic Constitution Pastor Aeternus of Pope Pius IX in 1870, chapter 4, near the end - the definition runs as follows:
“
And so We, adhering faithfully to the tradition received from the beginning of the Christian faith, to the glory of God, our Savior, the elevation of the Catholic religion and the salvation of Christian peoples, with the approbation of the sacred Council, teach and explain that the dogma has been divinely revealed: that the Roman Pontiff, when he speaks ex cathedra, that is, when carrying out the duty of the pastor and teacher of all Christians by virtue of his supreme apostolic authority he defines a doctrine of faith or morals to be held by the universal Church, through the divine assistance promised him in blessed Peter, operates with that infallibility with which the divine Redeemer wished that His church be instructed in defining doctrine on faith and morals; and so such definitions of the Roman Pontiff from himself, but not from the consensus of the Church, are unalterable.”
That is an infallibly true definition of doctrine, and it is infallibly true regardless of the reasoning for it, and regardless of the characteristics, quirks or failings of the Pope defining it. Those, however real, are not the foundation of the dogma, so they cannot be reasons to believe or to reject it. The human agency of the human teacher sinks down, as it were, behind the Authority of the Divine Teacher and Eternal Shepherd of the Church Who teaches through him. Dogmas are as undoubtedly certain as though they proceeded from the lips of Christ.
b. The Dogmatic Constitution Lumen Gentium of the Second Ecumenical Council of the Vatican, in 1964, paragraph 25.
Paragraph 25 runs as follows:
”
25. Among the principal duties of bishops the preaching of the Gospel occupies an eminent place.(39*) For bishops are preachers of the faith, who lead new disciples to Christ, and they are authentic teachers, that is, teachers endowed with the authority of Christ, who preach to the people committed to them the faith they must believe and put into practice, and by the light of the Holy Spirit illustrate that faith. They bring forth from the treasury of Revelation new things and old,(164) making it bear fruit and vigilantly warding off any errors that threaten their flock.(165) Bishops, teaching in communion with the Roman Pontiff, are to be respected by all as witnesses to divine and Catholic truth. In matters of faith and morals, the bishops speak in the name of Christ and the faithful are to accept their teaching and adhere to it with a religious assent. This religious submission of mind and will must be shown in a special way to the authentic magisterium of the Roman Pontiff, even when he is not speaking ex cathedra; that is, it must be shown in such a way that his supreme magisterium is acknowledged with reverence, the judgments made by him are sincerely adhered to, according to his manifest mind and will. His mind and will in the matter may be known either from the character of the documents, from his frequent repetition of the same doctrine, or from his manner of speaking.
Although the individual bishops do not enjoy the prerogative of infallibility, they nevertheless proclaim Christ's doctrine infallibly whenever, even though dispersed through the world, but still maintaining the bond of communion among themselves and with the successor of Peter, and authentically teaching matters of faith and morals, they are in agreement on one position as definitively to be held.(40*) This is even more clearly verified when, gathered together in an ecumenical council, they are teachers and judges of faith and morals for the universal Church, whose definitions must be adhered to with the submission of faith.(41*)
And this infallibility with which the Divine Redeemer willed His Church to be endowed in defining doctrine of faith and morals, extends as far as the deposit of Revelation extends, which must be religiously guarded and faithfully expounded. And this is the infallibility which the Roman Pontiff, the head of the college of bishops, enjoys in virtue of his office, when, as the supreme shepherd and teacher of all the faithful, who confirms his brethren in their faith,(166) by a definitive act he proclaims a doctrine of faith or morals.(42*) And therefore his definitions, of themselves, and not from the consent of the Church, are justly styled irreformable, since they are pronounced with the assistance of the Holy Spirit, promised to him in blessed Peter, and therefore they need no approval of others, nor do they allow an appeal to any other judgment. For then the Roman Pontiff is not pronouncing judgment as a private person, but as the supreme teacher of the universal Church, in whom the charism of infallibility of the Church itself is individually present, he is expounding or defending a doctrine of Catholic faith.(43*) The infallibility promised to the Church resides also in the body of Bishops, when that body exercises the supreme magisterium with the successor of Peter. To these definitions the assent of the Church can never be wanting, on account of the activity of that same Holy Spirit, by which the whole flock of Christ is preserved and progresses in unity of faith.(44*)
But when either the Roman Pontiff or the Body of Bishops together with him defines a judgment, they pronounce it in accordance with Revelation itself, which all are obliged to abide by and be in conformity with, that is, the Revelation which as written or orally handed down is transmitted in its entirety through the legitimate succession of bishops and especially in care of the Roman Pontiff himself, and which under the guiding light of the Spirit of truth is religiously preserved and faithfully expounded in the Church.(45*) The Roman Pontiff and the bishops, in view of their office and the importance of the matter, by fitting means diligently strive to inquire properly into that revelation and to give apt expression to its contents;(46*) but a new public revelation they do not accept as pertaining to the divine deposit of faith.(47*)”
http://www.vatican.va/archive/hist_c...entium_en.html
The same authentic doctrine of V2 is taught in the Catechism of the Catholic Church, paragraph 891.
http://www.scborromeo.org/ccc/p123a9p4.htm
Sorry about the length of this. All of that is meant only as information - not as apologetic for this dogma. It is very difficult to give clear info on this very controversial subject, without touching on a variety of other subjects.