Can a lay person or a group of lay faithful petition the Pope (Vatican) to de-canonize a Saint in light of new findings?
The short answer is yes.
But I doubt it will go very far. The canonization of a saint seems to imply the papal privilege of infallibility. But if one believes that they have a genuine case to the contrary, then one always free to petition the Holy See to reverse it’s decision!
If you wish you are always free to petition the Holy See with your suggestions and concerns. Any letter, should be tempered with genuine charity, respect and love for saving souls.
You can always write the Holy Father directly. The best way is to write your concerns in a letter and send it to the apostolic nuncio of your country.
The letter should be sent inside a larger envelope and with a outer letter requesting that it be given to the Supreme Pontiff via the next attaché pouch going to Rome.
Fair warning: Such letters should be in an unsealed envelope, as the apostolic nuncio’s secretariat must read all correspondence destined to be handed over to the Pope in person. This I know through personal experience.
Not many Catholics have personal ties and friendships with cardinals or Vatican officials to circumvent this step.
Go ahead and suggest your ideas in a letter to the Pope. Warning once more: Remain courtesy, polite, logical, and politically, scientifically, morally and theologically sound.
The People of God may manifest their concerns about the actions of the Holy See to their pastor, bishops and even the pope as the Supreme Pastor. This is backed up by Canon Law:
Can. 212 §1. Conscious of their own responsibility, the Christian faithful are bound to follow with Christian obedience those things which the sacred pastors, inasmuch as they represent Christ, declare as teachers of the faith or establish as rulers of the Church.
§2. The Christian faithful are free to make known to the pastors of the Church their needs, especially spiritual ones, and their desires.
§3. According to the knowledge, competence, and prestige which they possess, they have the right and even at times the duty to manifest to the sacred pastors their opinion on matters which pertain to the good of the Church and to make their opinion known to the rest of the Christian faithful, without prejudice to the integrity of faith and morals, with reverence toward their pastors, and attentive to common advantage and the dignity of persons.
As I stated before, I truly believe there will be no de-canonization that will result in such a petition or dubia. The reason being is that many Catholics believe that the authority to canonize someone a saint is intimately tied to the privilege of papal infallibility.
First, let me say that the word define (definimus) is used in the original Latin, and the Pope is thus exercising his authority.
Second, the object of canonization is that the person declared as a saint is now in heaven and can be invoked as an intercessor by all the faithful. The infallibility of this action is accepted by the majority of Catholic theologians but has not itself been the subject of a definition.
Thus, with the act of canonization the Pope, so to speak, imposes a precept upon the faithful by saying that the universal Church must henceforth keep the memory of the canonized with pious devotion.
The 1967 New Catholic Encyclopedia discusses the theological foundation for the infallibility of canonization: "The dogma that saints are to be venerated and invoked as set forth in the profession of faith of Trent (cf. Denz. 1867) has as its correlative the power to canonize. ... St. Thomas Aquinas says, 'Honor we show the saints is a certain profession of faith by which we believe in their glory, and it is to be piously believed that even in this the judgment of the Church is not able to err' (Quodl. 9:8:16).
"The pope cannot by solemn definition induce errors concerning faith and morals into the teaching of the universal Church. Should the Church hold up for universal veneration a man's life and habits that in reality led to [his] damnation, it would lead the faithful into error. It is now theologically certain that the solemn canonization of a saint is an infallible and irrevocable decision of the supreme pontiff. God speaks infallibly through his Church as it demonstrates and exemplifies its universal teaching in a particular person or judges that person's acts to be in accord with its teaching."
At the same time, it is important to note that while the decree of heroic virtues and the miracle form a necessary part of the process of canonization, they are not the specific object of the declaration of infallibility.
Although the saint is proposed as a model of virtues and Christian living, it is not the specific object of canonization. For example, it is quite possible that a martyr show heroic virtue in the face of death without necessarily having lived all the virtues to an exemplary degree. Nor does canonization make the saints immune from the judgment of history insofar as hindsight might show that some of their external actions proved to be unwise or had negative consequences.
This argument therefore would place the infallibility of canonization within the area of faith insofar as the venerability of saints is a dogma grounded in Revelation, and the determination as to which persons can be thus venerated is a necessary exercise of infallible authority.
This is sometimes called the secondary object of infallibility. It is not revealed dogma per se but truths regarding faith and morals which are not formally revealed but are so bound up with divine Revelation that to deny them would lead us to many difficulties and even lead to a denial of some aspect of Revelation itself.
According to Ludwig Ott's classical manual of dogmatic theology there are four kinds of teaching involved in this exercise of infallibility: Theological conclusions derived from formally revealed truths by aid of the natural truth of reason; historical facts on the determination of which the certainty of a truth of Revelation depends (so-called "dogmatic facts," for example, "Is Pope N. truly the duly elected and rightful successor to the throne of Peter?"); natural truths of reason which are intimately connected with Revelation (e.g., the morality of certain medical procedures); the canonization of saints (see Fundamentals of Catholic Dogma, p. 299).
A further argument can be offered. With a canonization, the Pope mandates (rather than permits, as is the case of beatification) that a saint be venerated in the Church's liturgy and especially with the Eucharistic celebration in his honor. Considering that the Mass is the highest and most perfect form of worship, it is logical that the Holy Spirit would guard the Pope and the Church from any error regarding a canonized person's definitive state. At the same time, it must be recognized that this is an argument based on congruence and is not apodictic. The institution of a liturgical celebration does not in itself imply an exercise of infallibility. - Canonizations and Infallibility
In any case, a lay person or a group of lay individuals may petition or even present a dubium to the pope through the proper ecclesiastical channels.
A dubium is simply a request asking for clarity on this issue or some ecclesiastical matter.
The Church is not in the habit of de-canonizing her validly canonized saints. After all, we are not followers of Henry the VIII, who de-canonized St. Thomas à Becket in 1538!