Let’s just clear up this assertion first. Pope St. John Paul II did not know that McCarrick was abusing seminarians. There had been accusations against him at the time, but he swore to Pope St. John Paul II that the accusations were not true. Not only that, but several other bishops who were close to McCarrick also told the saintly Pope that the accusations were false. Can one fault Pope St. John Paul II for believing them?
I know little to nothing of Pope St. John XXIII, so can’t comment on his saintliness. Pope St. Paul VI, however, was greatly admired by other saintly men in his time. Ven. Fulton Sheen, for example, shortly after Paul VI’s death, publicly said he believe Paul VI should be canonized because, having known him personally, he knew how saintly he was.
As far as these “bullet train” (I like that phrase ) canonizations are concerned, they’re not completely without precedent. To my knowledge, St. Francis of Assisi still holds the record for fastest canonization (I believe within a year of his death).
I think we often make the mistake of believing that because a saint is canonized he or she must’ve been perfect (without flaw, never made a mistake, never had a lapse in judgement, etc.). This is simply not the case. Some saints made some very egregious mistakes. As an examples, St. Cyril of Alexandria, in his zeal to combat what he perceived to be heresy, failed to fully comprehend Nestorius’s theology, gathered a council that condemned Nestorius without giving him the chance to defend his own theology, and ultimately created a schism in the Church that lasts to today. I’d call that a whopper of a mistake.
Saints aren’t perfect. They are simply sinners like you and I, but they are sinners who continued on the path of repentance throughout their life. I love the story of a young monk who encountered a desert hermit. He asked the hermit, “What do you do out here in the desert?” The hermit responded, “I fall down and I get back up. I fall down again, and I get back up.” That pretty much sums up the lives of the saints.