I am returning to the stated topic of this thread.  I do not know if the report is true, but if it is true, then the person in question -- or anyone walking a similar path -- was poorly catechized to begin with.  

The Novus Ordo has become, correctly, an umbrella term.  Used without the third word, "Mass," (appropriately in quotation marks, some would say).  "The Novus Ordo" is not just a worship "form."  It is a belief system, harmonious with that same "liturgy." It is by definition a false belief system, because there is no authentic Catholic Church that is 58 years old.  There's no such thing as "a modern Catholic Church."  It remains ancient in its origins, or it is a different church.

It's a vague term, so people should be more careful when using it.  For many Novus Ordo means the 1969 missal, for others it includes the traditional masses said with permission of the diocese, for others, as you correctly state, it's the belief system.   

The question thus becomes whether Pope Saint Pius V had the authority to declare the Roman liturgical rite, the rite that was given primacy for the Western Church at Trent, irreformable for all time.

Fr. Wathen makes a very strong case for it in his book The Great Sacrilege.

He clearly didn't even intend to, seeing as he changed it himself later. The commands were directed at lower-ranking clergy, i.e it was banning any patriarchs, bishops etc. altering the rite in their own regions.

The substance of the Mass is not to be changed by altering the rites. That's not the same as unsubstantial changes. The new order rites and service (erroneously called "mass") are substantial changes.

The substance of the Mass is not to be changed by altering the rites. That's not the same as unsubstantial changes. The new order rites and service (erroneously called "mass") are substantial changes.

Quo Primum makes no distinction. It doesn't imply or state it permits "unsubstantial changes".

Again, putting words in Pope St. Pius V's mouths to justify a nonsensical and legally impossible interpretation of the bull.

Going by your "logic", you must not believe in the Holy Trinity because the bible doesn't specifically state the Name, nor do you believe in the Immaculate Conception, the Assumption, or the Coronation, because they aren't specifically stated in the bible. Forget holy tradition, we have to take a literalist word-for-word view of written precepts whether it's canon or papal decrees.

Be fair, Tourmalet. You are pushing your argument too far, specifically in that the Bible and Tradition are simply irrelevant in the present context. That is, the matters you cite are all dogmas, not matters of papal jurisdiction—or rather, each one lost any jurisdictional dimension it might ever have had once it was dogmatically defined.

I simply do not know enough of the precise circuмstances surrounding the drafting of Quo Primum to weigh in one way or the other on forlorn's assertion regarding the bull's primary target audience. His assertion does, however, have the great merit of being prima facie defensible, which is more than can be said for assertions regarding the bull's permanently irreformable character. That is to say, it is by no means plain from the language of the bull that it indissolubly links the celebration of the Roman rite with the content of the Faith as defined for all time.

Surely the plain meaning of the power of the Keys is that it confers upon each and every pope enormous latitude to act as he chooses, for good or for ill, while being answerable to God alone—save in the few exceptional circuмstances whose character is so resistant to definition that Bellarmine and other authorities cannot even agree upon what those circuмstances are or what options are available to those who would counter the abuse!

As someone who buys into Father Scott's argument that it is not licit to avail oneself of the Mass promulgated for the Church by someone who is at the same time recognized as a true pope, your lack of doubt in that regard doesn't surprise me. Your adoption of Father Scott's "not licit" argument is as manifest a condemnation of your "judgment" as the wearing of scarlet letter formerly was of moral turbitude turpitude.

Beneath all this pompous blather, one detects the following: "Cool people understand that the SSPX position, expressed by Father Peter Scott, is self-evidently wrong, and your accepting it demonstrates that you are a jerk, claudel." Unfortunately, you probably would have misspelled jerk, too.

For once in your life, do something useful and appropriate: namely, explain the basis on which you declare the position of the SSPX and Father Scott wrong and show precisely how the rationale that underlies the "valid but not licit" position offends Faith, reason, or both.

"Turbitude"? Really?

As per Quo Primum

"[...] whereas, by this present Constitution, which will be valid henceforth, now, and forever, We order and enjoin that nothing must be added to Our recently published Missal, nothing omitted from it, nor anything whatsoever be changed within it under the penalty of Our displeasure.

"[...] Therefore, no one whosoever is permitted to alter this notice of Our permission, statute, ordinance, command, precept, grant, indult, declaration, will, decree, and prohibition. Would anyone, however, presume to commit such an act, he should know that he will incur the wrath of Almighty God and of the Blessed Apostles Peter and Paul."

We only need to look at the counter church, today and for the 51 years, to see this very wrath upon her.

https://www.papalencyclicals.net/Pius05/p5quopri.htm

"nor anything whatsoever be changed"

not "nor anything whatsoever substantial be changed".

I don't know what you thought you achieved by reposting quotes I already gave. They still contradict your ridiculous position.

So you don't believe in the Holy Trinity? The Name Holy Trinity is mentioned nowhere in the bible.

I can show you it defined dogmatically, if you'd like. Can you show me where it says Quo Primum only prohibits substantial changes, besides your imagination?