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.