Actually, Quo Primum lays out the rules quite clearly.

1.  The latin rite of St Pius V can be used in perpetuity (allowance/permission to use).
2.  No one can change, alter or add anything to the rite (no changes to the rite at all).
3.  ONLY this rite can be used.  No other rites are allowed (strict use of this rite is commanded).

**Exceptions exist for ancient rites (i.e. Benedictine/Dominican) within the latin rite.  Except for a few prayers, these rites are nearly identical to the latin rite.

The loophole in Quo Primum, is that it never forbid a future pope from creating a new rite (which V2 did); but it does forbid it's use, in 2 ways - it says only the latin rite can be used and the latin rite MUST be used.  So it commanded such from 2 different angles, to make it clear.  But the Modernists found the loophole which allowed them to create a new rite (which is technically legal), and then they used half-truths, lies, manipulation and pressure from bishops to push people into accepting this illegal rite.

58. 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.

Pius XII said this in his encyclical about the Mass, Mediator Dei:

58. 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.

When it comes to the Liturgy of the Roman Rite, really the only legal leg that trads have to stand on is the fact that Quo Primum is binding in perpetuity. Because we are bound, the pope has to be bound - there is no way out of this, there simply isn't.

Had PPV invented his own new rite, then maybe his successors would not be bound. But that's not the case here.

Here the liturgy was restored to the original from the time all the way back to the Apostles.

The purpose of binding us was for the sake of unity, so that all the "priests would know which prayers to use and which rites and ceremonies they were required to observe from now on in the celebration of Masses."

Yes, as it stands, since none of the V2 "popes" changed Quo Primum, then they have to abide by it.  It's also a very important point that neither Paul6 or any other V2 "pope" made the new mass obligatory, or binding under pain of sin.  Because they knew that Quo Primum forbid it.  And Benedict XVI confirmed this in his motu where he said that QP "was never abrogated (i.e. ended)" and thus the True Rite/Mass "was always allowed."
Any pope can create his own rite (in theory), just like they can create new missals.  The point is, the new rite or missal must be Apostolic/Traditional and valid (i.e. the essence of the rite must be catholic).

Each catholic rite is essentially the same, because they all originated from the Apostles, who learned from Christ.  So if a pope were to create a new rite, there wouldn't be much of a difference in substance, only in minor things. 
Yes and no.  St Pius V restored *some* things back to the time of St Gregory the Great (600s) but he also accepted a great many things which were added/created during the 1,000 year period of Christendom, from the 500s to the 1,500s.  Example:  St Thomas Aquinas' hymns used at Benediction, the many Litanies, etc.

The natural evolution of the liturgy of these 1,000 years was a necessary and beautiful part of the Holy Ghost's perfection of the rite.  But the things added were not essential, only minor.
Yes, there were so many things added during this 1,000 years that it got out of hand.  So St Pius V was just saying, "Ok, everyone, the last 1,000 years have been very creative and beautiful.  Some of you have gone a little overboard with the additions, and the masses in England vs France are just too different.  We are losing Catholic unity.  So i'm going to pick the best ideas and this new missal is the one everyone should use from now on.  No more changes."

the 1568 docuмent "Quod a Nobis" which introduced the new Roman breviary two years before the new missal.

but perhaps someone who knows Latin can compare the two bulls and give some creditable review of the claim about both bulls having similar language.

The allowance of minor changes, by future popes, goes without saying.  Because minor changes aren't real changes.  They don't affect doctrine or the essence of the liturgy.

Montini explicitly overrode it when he promulgated the NOM.  You don't edit "Quo Primum," but you modify it or abrogate it with subsequent legislation.  What is this "modifying"?  No pope directly modifies a previous pope's docuмent.

Montini explicitly overrode it when he promulgated the NOM.

You don't edit "Quo Primum," but you modify it or abrogate it with subsequent legislation.  What is this "modifying"?

No pope directly modifies a previous pope's docuмent.