Stubborn is now promoting the principles of Antiquarianism, which is precisely what the Liturgical Deformers at the time of Vatican II did.  It just keeps getting better, as we watch him cut off his nose to spite his face.

Pius XII, Mediator Dei:

The sacred liturgy does, in fact, include divine as well as human elements. The former, instituted as they have been by God, cannot be changed in any way by men. But the human components admit of various modifications, as the needs of the age, circuмstance and the good of souls may require, and as the ecclesiastical hierarchy, under guidance of the Holy Spirit, may have authorized.
...
It follows from this that the Sovereign Pontiff alone enjoys the right to recognize and establish any practice touching the worship of God, to introduce and approve new rites, as also to modify those he judges to require modification.

You should read the entirety of that Encyclical, since it debunks 90% of the nonsense you've been spewing here.

Dear Mr. Intellectual, you have no idea what you're talking about.

It is apparent this needs to be explained to you.

You can quote popes all day long stating that they can :
"...introduce and approve new rites, as also to modify those he judges to require modification."

And I can quote Pope St. Pius V all day long stating that nobody of "whatever ecclesiastical dignity" is permitted to add, omit or change anything in the missal. If anything ever needed to be modified, in Quo Primum he said he already did it. The law of Quo Primum is to preserve that missal. Period.

How then could Pope Clement VIII issue this bull in 1604 changing the same Missal of St Pius V?

Did you read the Bull? He says through the years the missal had many errors due to printing or whatever, so he re-edited the corrupted missal so that is was put back to the original of Pope St. Pius V. He did not invent a new rite or change it.

From your link....."Although he very severely forbade under many penalties that anything should be added to it, or that anything for any reason be removed from it, nevertheless, in the course of time, it has come to pass that, through the rashness and boldness of the printers, or of others, many errors have crept into the missals which have been produced in recent years....

...Having considered these innovations, in Our pastoral solicitude which induces us to earnestly protect and preserve in everything and especially in the sacred rites of the Church the best and old norm, We have ordered in the first place that the above mentioned printed Missals, so corrupted, be banned and declared null and void and that their use be disallowed in the celebration of the Mass, unless they be entirely and in everything emended according to the original text published under Pius V...."

There are differences between the Missals of St Pius V, Clement VIII and Urban VIII.

To give an example:

In the 1570 Missal after the Words of Consecration of the chalice the priest elevates the chalice whilst saying Haec quotiescuмque whilst elevating the chalice.
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In the Clement VIII Missal the rubric is changed and the celebrant says Haec quotiescuмque after placing the chalice back on the corporal and before genuflecting and then elevating the chalice:
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However, Clement VIII continues:

"We have also entrusted some of Our Venerable Brethren, Cardinals of the Holy Roman Church, versed in Holy Writ and skilled in ecclesiastical antiquity, with the business of restoring the Missal to its primitive and purest form. In their loyalty to Us, and in their piety and devotion to the Church, these Cardinals, employing also other learned men trained in ecclesiastical scholarship and having searched for, and diligently examined, old Missals and other books bearing upon the subject, have endeavored to restore the Roman Missal to its original purity and to confirm and attest the painstaking care and diligence of Pius V and of those appointed by him. It happened, however, that in the carrying out of this task, as a result of an accurate comparison of ancient books, some things have been improved upon and, concerning the rules and rubrics, some points have been more fully and clearly stated. These improvements, however, flowing as it were from the same sources and principles, seem rather to represent and complete the meaning of the rules and rubrics than to introduce anything new.

"We have ordered, therefore, that the Missal which the same Pius V had edited, thus revised, be printed as faultlessly as possible at Our Vatican printing press and that it be published for the common benefit."

Again, the only reason he did anything at all to the Missal was because through the years the missal had many errors due to printing or whatever. He did not invent a new rite or change the old rite. Your above quote even states: "These improvements, however, flowing as it were from the same sources and principles, seem rather to represent and complete the meaning of the rules and rubrics than to introduce anything new."

The idea that the language used for Quo Primum is merely the theatrical norm used in those days and on that account QP does not mean what it says, is 100% Novus Ordo approved thinking.
The idea that a pope can change, or even totally discard the liturgy of the Roman Rite because popes cannot bind popes, is also 100% Novus Ordo approved thinking.

It is NO approved thinking because that is the NO thinking the modernist enemies of the Church managed to convince so many of the faithful of when this crisis was in it's infancy. It went something like this:
"Why the changes?"
"Because he's the pope, popes cannot bind other popes," and "The pope can do whatever he wants." - This is the approved NO thinking I am talking about. Understand that that answer settled the matter for multitudes of otherwise formerly faithful Catholics who went NO because the pope cannot be bound by his predecessors. This means PPVI had all the authority he needed to do what he did and everyone had to obey.

Further, re: the docuмents of V2 - the same trads who claim QP does not mean what it says, roundly condemn V2 for meaning what they say.  Why?

Perhaps it is better to ask, then, if Pius V had never issued Quo primum, what would the status of the traditional Roman rite be?

We'd have what we have now - because PPVI ignored Quo Primum as if it was never issued and did not exist.

If the pope has no authority to create a new rite, then Quo primum is irrelevant.

No, PPV never created a new rite - he restored it to it's original. The Law of Quo Primum was established to protect the Mass forever, to forever preserve the Liturgy he restored - just as it says in Quo Primum.

We'd have what we have now - because PPVI ignored Quo Primum as if it was never issued and did not exist.

No, PPV never created a new rite - he restored it to it's original. The Law of Quo Primum was established to protect the Mass forever, to forever preserve the Liturgy he restored - just as it says in Quo Primum.

You misunderstand me. Let me rephrase: If Pius V had not issued Quo primum, would there be any grounds for a priest to refuse the NOM? In other words, is the right to the traditional Mass only based in Quo primum, or does the right exist indepedent of Quo primum?

Furthermore, St. Pius V did not restore the rite to "its original," which is patently false from a historical point of view, nor did he claim to. The Latin says: ad pristinam Missale Sanctorum Patrum normam ac ritum restituerunt. It does not say "the original;" the word is "pristine." Moreover, the Holy Fathers referred to are not what we call the "Church Fathers." Rather, it refers to more recent times. This can be confirmed if you compare the 1570 Missale with the 1474 printing.

We need this for establishing where the Canon truly ends, and this will help with understanding what the Canon is.

No, we don't need this.  Others have cited authoritative sources about where the Canon ends, nor does it really matter, as it's clear that a Pope has the authority to make changes to the Canon.