It is correct that the concise and true answer is that all questions about the legitimacy of the present pontificate are governed entirely by what Pope John Paul II wrote and published in the contents of the Apostolic Constitution, Universi Dominici Gregis.  While Universi Dominici Gregis may happen to repeat what some previous Pope legislated about conclaves, the fact that a previous Pope made an even identical rule, is more or less meaningless.  Universi Dominici Gregis integrates in one document the applicable law, and replaces whatever Popes legislated previously about these matters, including once valid and licit single sentences in a constitution or other single statements made by a previous Pope, as clearly provided in the promulgation clause at the end of Universi Dominici Gregis (Footnotes omitted):

“PROMULGATION

“Wherefore, after mature reflection and following the example of my Predecessors, I lay down and prescribe these norms and I order that no one shall presume to contest the present Constitution and anything contained herein for any reason whatsoever. This Constitution is to be completely observed by all, notwithstanding any disposition to the contrary, even if worthy of special mention. It is to be fully and integrally implemented and is to serve as a guide for all to whom it refers.

“As determined above, I hereby declare abrogated all Constitutions and Orders issued in this regard by the Roman Pontiffs, and at the same time I declare completely null and void anything done by any person, whatever his authority, knowingly (scienter) or unknowingly (inscienter), in any way contrary to this Constitution.”

Just because a prelate, priest or another member of the faithful may quote from what a previous Pope stated does not make what that previous Pope stated apply to the organization, conduct or significance of what may have occurred at a conclave governed by Universi Dominici Gregis.  Pope John Paul II in Universi Dominici Gregis clearly intended to supplant entirely whatever any previous Pope may have provided about these matters.  This understanding of the vast sweep of the Universi Dominici Gregis is especially clear when one considers the preambulary language of Pope John Paul II in Universi Dominici Gregis:“Precisely for this reason, down the centuries the Supreme Pontiffs have deemed it their special duty, as well as their specific right, to establish fitting norms to regulate the orderly election of their Successor. Thus, also in more recent times, my Predecessors Saint Pius X, Pius XI, Pius XII, John XXIII and lastly Paul VI, each with the intention of responding to the needs of the particular historical moment, issued wise and appropriate regulations in order to ensure the suitable preparation and orderly gathering of the electors charged, at the vacancy of the Apostolic See, with the important and weighty duty of electing the Roman Pontiff.
“If I too now turn to this matter, it is certainly not because of any lack of esteem for those norms, for which I have great respect and which I intend for the most part to confirm, at least with regard to their substance and the basic principles which inspired them. What leads me to take this step is awareness of the Church’s changed situation today and the need to take into consideration the general revision of Canon Law which took place, to the satisfaction of the whole Episcopate, with the publication and promulgation first of the Code of Canon Law and subsequently of the Code of Canons of the Eastern Churches. In conformity with this revision, itself inspired by the Second Vatican Ecumenical Council, I then took up the reform of the Roman Curia in the Apostolic Constitution Pastor Bonus.7 Furthermore, Canon 335 of the Code of Canon Law, restated in Canon 47 of the Code of Canons of the Eastern Churches, makes clear the need to issue and constantly update the specific laws regulating the canonical provision for the Roman See, when for any reason it becomes vacant.

“While keeping in mind present-day requirements, I have been careful, in formulating the new discipline, not to depart in substance from the wise and venerable tradition already established.

“It is in fact an indisputable principle that the Roman Pontiff has the right to define and adapt to changing times the manner of designating the person called to assume the Petrine succession in the Roman See. .   .   .” 

“Finally, I have deemed it necessary to revise the form of the election itself in the light of the present-day needs of the Church and the usages of modern society.”

Thus, the Cardinals and all the faithful must accord unique significance to what Pope John Paul II stated as universal law, including what he legislated in Paragraph 76 of Universi Dominici Gregis: “Should the election take place in a way other than that prescribed in the present Constitution, or should the conditions laid down here not be observed, the election is for this very reason null and void, without any need for a declaration on the matter; consequently, it confers no right on the one elected.”

With the exception of Paragraph 55 which requires that “every effort to preserve that secrecy by ensuring that no audiovisual equipment for recording or transmitting has been installed by anyone”, yet provides that: “Should any infraction whatsoever of this norm occur and be discovered, those responsible should know that they will be subject to grave penalties according to the judgment of the future Pope.”, because both Paragraph 76 and the “Promulgation” language at the end of Universi Dominici Gregis apply universally, any man once having been a valid Cardinal who otherwise (except for Paragraph 55) violates Universi Dominici Gregis instantly loses that status as a valid member of the College of Cardinals by operation of law without any further judgment or declaration on the matter, and is therefore no longer a valid Papal elector, regardless what any previous Pope legislated or stated on the matter.  This is abundantly clear also because of the use of the Latin word “scienter” in the promulgation of the Latin original of Universi Dominici Gregis, a word of great legal significance and logical import never used by another Pope in any previous apostolic constitution on these matters.  

That is why it is so important for those valid Cardinals to unite and to deal with the many invalid cardinals.  If those valid cardinals determine that the supposed conclave election of Monsignor Bergoglio was “null and void”,  then they should determine to the extent possible a new and proper list of valid cardinals, likely a daunting task which requires much prayer and grace.  The reason so many Cardinals accept the validity of the last Conclave may be that a majority of those who are age-eligible as Electors were appointed invalidly since the resignation of Pope Benedict XVI and the ensuing null and void conclave in March 2013.

Unless and until unquestionably true and valid Cardinals review and determine whether the last conclave was a nullity, the whole canonical apparatus for continuing the office of Successor of Saint Peter is in jeopardy.  If that last conclave was a nullity, then no valid cardinals have been appointed since those last appointed by Pope Benedict XVI and Pope John Paul II.  When those cardinals pass the age limit, absent a miracle, “All the King’s horses and all the King’s men” won’t be able to put the Papacy back together again.  As referenced by Cardinal Eijk, you may wish to read the Catechism of the Catholic Church, ¶¶ 675, 676 & 677.  While we are guaranteed an hierarchy sufficient to provide the Sacraments until The End, The Church is not guaranteed to have a Pope until The End.  Directing these matters to anyone other than valid Cardinals (and determining who those men are is problematic) is dangerous and naive.  Most of the work that good Cardinals could do on this Bergoglian problem would not be public, at least until they reach a conclusion.   If they were to determine nullity, then, absent miraculous Divine intervention, the valid cardinals would likely need the support of a major secular power to enforce their determination and carry out a proper conclave.  That is the reality.  All of the papal heresy discussion may just provide “backhanded” validation of the last conclave.  The cardinals may have unwittingly experienced certain invalidating violations of Universi Dominici Gregis which made that conclave a total nullity.  If so, those cardinals “taking a tack” that entails claims of heresy by a valid Pope is fraught with potential for a huge, if not permanent, disruption of The Church.  (Even good Cardinals may have engaged in this approach, apparently suffering from “perfect report card” syndrome, never wanting to admit that they were even a small part of a major mistake.)