As for the Pseudo-Isidorian Decretals and the Donation of Constantine, both were indeed forgeries or --- what's worse --- mixed collections of forged and genuine manuscripts. Most of what Pseudo-Mary-Ann-Collins has to say about them is false or --- what's worse --- a mixture of tendentious and genuine interpretation.
(Pause here for all characters to change their names and reappear different costumes like a fast-paced Shakespearean farce.)
The False Decretals were evidently concocted by some Frankish monks in the 800's as an attempt to get the Carolingian Empire to back off on trying to dominate the bishops, either directly as the Empire bossing around the Church, or indirectly as imperial allies (archbishops and so forth) trying to boss around little local bishops and abbots.
In many ways an admirable project --- monks trying to defend the liberty of their local abbot --- though not, of course, by forgery.
The people through the centuries who quoted from these False Decretals (including Aquinas) were emphatically not frauds: they were just people who thought them to be genuine manuscripts, as did everybody at the time (i.e. everybody on both sides of many a disputed question.)
The person who finally did do the necessary, painstaking detective work was none other than a fifteenth century Latin scholar, Cardinal Nicholas of Cusa. A Catholic, you'll note. It's been a long time since anybody seriously argued that the Decretals and the Donation were genuine.
So the Pseudo-Mary-Ann runs up now to the train station, red-faced but triumphant, proclaiming that she has discovered these forged Frankish manuscripts?
That train left a couple centuries ago, Pseudo-Sister. Nice try.
It is also shown that Peter is the Vicar of Christ and the Roman Pontiff is Peters successor enjoying the same power conferred on Peter by Christ. For the canon of the Council of Chalcedon says: If any bishop is sentenced as guilty of infamy, he is free to appeal the sentence to the blessed bishop of old Rome, whom we have as Peter the rock of refuge, and to him alone, in the place of God, with unlimited power, is granted the authority to hear the appeal of a bishop accused of infamy in virtue of the keys given him by the Lord. And further on: And whatever has been decreed by him is to be held as from the vicar of the apostolic throne.
Likewise, Cyril, the Patriarch of Jerusalem, says, speaking in the person of Christ: You for a while, but I without end will be fully and perfectly in sacrament and authority with all those whom I shall put in your place, just as I am also with you. And Cyril of Alexandria in his Thesaurus says that the Apostles in the Gospels and Epistles have affirmed in all their teaching that Peter and his Church are in the place of the Lord, granting him participation in every chapter and assembly, in every election and proclamation of doctrine. And further on: To him, that is, to Peter, all by divine ordinancebow the head, and the rulers of the world obey him as the Lord Jesus himself. And Chrysostom, speaking in the person of Christ, says: Feed my sheep (John 21:17), that is, in my place be in charge of your brethren" (St. Thomas Aquinas, Against the Errors of the Greeks. Found in James Likoudis, Ending the Byzantine Greek Schism (New Rochelle: Catholics United for the Faith, 1992), pp. 182-184).
With the exception of the last reference to Chrysostom all of Thomas references cited to Cyril of Jerusalem, Cyril of Alexandria, Chrysostom and the Council of Chalcedon are forgeries. The remainder of Aquinas treatise in defense of the papacy is similar in nature. Edward Denny gives the following historical summary of these forgeries and their use by Thomas Aquinas
In the Pseudo-Isidorian Decretals alone 313 of the 324 claims attributed to your religion are false; lies...The question is, how many of these 313 lies does the modern Catholic church still teach as truth, even knowing they are lies???
Though I can appreciate the Catholic motivation to brush aside whatever came about as a result of forgeries in the Church during the first fifteen hundred years and, of course, any messenger who happens to dare mention them, what I don't see is any attempt to undo the many bulls and dogmas, canons and credos that resulted from those now-proven false manuscripts. Any Catholic cleric pursuing that painstaking detective work that you know of?