I watched this video for the first time today.

In the NOW voice heard on this video, by adding to what Paul VI said not quoted by Taylor Marshall,  the voice puts forward the argument that once a pope decrees something, as it is from the magisterium, all Catholics must adhere to it. Am I right?

OK then, let us put the NOW voice to the test.

In 1616, Pope Paul V decreed:
(1) “That the sun is in the centre of the world and altogether immovable by local movement,” was unanimously declared to be “foolish, philosophically absurd, and formally heretical [denial of a revelation by God] inasmuch as it expressly contradicts the declarations of Holy Scripture in many passages, according to the proper meaning of the language used, and the sense in which they have been expounded and understood by the Fathers and theologians.”

Now, Trent and Vatican I defined that when all the Fathers hold to a Biblical revelation (that it reveals the sun and stars run a course) then it must be held as irreformable.

In 1633, the sentence against Galileo said:
“Invoking, then, the most holy Name of our Lord Jesus Christ, and that of His most glorious Mother Mary ever Virgin, by this our definitive sentence we say, pronounce, judge, and declare, that you, the said Galileo, on account of these things proved against you by docuмentary evidence, and which have been confessed by you as aforesaid, have rendered yourself to this Holy Office vehemently suspected of heresy,  that is, of having believed and held a doctrine which is false and contrary to the sacred and divine Scriptures - to wit, that the sun is in the centre of the world, and that it does not move from east to west, and that the Earth moves, and is not the centre of the universe; and that an opinion can be held and defended as probable after it has been declared and defined to be contrary to Holy Scripture.

Then, in 1820, pope Pius VII, convinced a solar-system had been proven and that the 1616 and 1633 decrees were wrong, he decreed:
'His Holiness has decreed that no obstacles exist for those who sustain Copernicus’ affirmation regarding the Earth’s movement in the manner in which it is affirmed today [fixed-sun/moving Earth], even by Catholic authors.

In 1822 Pope Pius VIII further decreed;
‘The most excellent [Holy Office] have decreed that there must be no denial, by the present or by future Masters of the Sacred Apostolic Palace, of permission to print and to publish works which treat of the mobility of the Earth and of the immobility of the sun [the defined heresy in 1616], according to the common opinion of modern astronomers, as long as there are no other contrary indications, on the basis of the decrees of the Sacred Congregation of the Index of 1757 and of this Supreme [Holy Office] of 1820; and that those who would show themselves to be reluctant or would disobey, should be forced under punishments at the choice of [this] Sacred Congregation, with derogation of [their] claimed privileges, where necessary.’

Then, on 20 May 1833, while deliberating on a new proposed edition of the Index, Pope Gregory XVI decided that it would omit the five [1616 banned] books by Galileo, Copernicus, Kepler, Foscarini and Zúñiga, but that this omission would be made without explicit comment. Gregory XVI is the pope who taught the following:
 
“Therefore, the Church has, by its divine institution, the power of the magisterium to teach and define matters of faith and morals and to interpret the Holy Scriptures without danger of error.”--- Commissum Divinitus (#4).

Next Vatican I;
‘But since the rules which the holy Synod of Trent salutary decreed concerning on the interpretation of Divine Scripture in order to restrain impetuous minds, are wrongly explained by certain men, We renewing the same decree, declare this to be its intention: that in matters of faith and morals pertaining to the instruction of Christian Doctrine, as must be considered as the true sense of Sacred Scripture which Holy Mother Church has held and holds, whose office it is to judge concerning the true understanding and interpretation of Sacred Scripture; and, for that reason, no one is permitted to interpret Scripture itself contrary to this sense, or even contrary to the unanimous agreement of the Fathers.’--- (Vatican 1, Denz. 1788)

But along comes Pope Leo XIII' Providissimus deus:
‘18: The unshrinking defence of the Holy Scripture, however, does not require that we should equally uphold all the opinions which each of the Fathers or the more recent interpreters have put forth in explaining it; for it may be that, in commenting on passages where physical matters occur, they have sometimes expressed the ideas of their own times [like sunset and sunrise], and thus made statements which in these days have been abandoned as incorrect. Hence, in their interpretations, we must carefully note what they lay down as belonging to faith, or as intimately connected with faith, what they are unanimous in. For “in those things which do not come under the obligation of faith, the saints were at liberty to hold divergent opinions, just as we ourselves are,” according to the saying of St. Thomas Aquinas.’

Then along came Einstein admitting THE CHURCH OF 1616 AND 1633 HAD NEVER BEEN PROVEN WRONG.

Two more encyclicals on the interpretation of Scripture by Pope Benedict XV and Pius XII repeated the idea that the Bible was interpreted wrong by Pope Paul V in 1616 and Pope Urban VIII in 1633.

Finally on to VATICAN II.

‘The humble and persevering investigator of the secrets of nature is being led, as it were, by the hand of God in spite of himself, for it is God, the conserver of all things, who made them what they are. We cannot but deplore certain attitudes (not unknown among Christians) deriving from a short-sighted view of the rightful autonomy of science; they have occasioned conflict and controversy and have misled many into opposing faith and science.’ --- Gaudium et spes, # 36.

So, here we have popes since 1820 contradicting one another and Vatican II accusing its predecessors of making mistakes in their decrees and placing the autonomy of science ABOVE the autonomy of Scripture.

So much for the NOW voice telling us in this video we must accept everything what popes teach.
 It was this passage in Vatican II that GUARANTEES it was not a Council any Catholic could take seriously.

I'm having a hard time reading right now and I don't fully trust these guys, but is there info in this article which helps clarify the matter?

The Holy Office - Is It Infallible? (Geocentrism & Fr. Feeney)

It's from Most Holy Family Monastery

The whole mess with Heliocentrism, The Gallileo Affair, and all the problems that resulted from it later after The Church changed its stance on Heliocentrism is the reason some Sede's think the See has been vacant since 1740. 

Its actually the opposite EWPJ. Sede's must argue the 1616 and 1633 decrees were not of the magisterium because that would make all popes since 1820 heretics. Now that is not an option for sedes so they must deny the infallibility of the decrees against biblical heliocentrism.

As I said above, the Dimond Brothers's sedevacantism depends on their rejecting the 'Infallibility' of the 1616 decree defined and decreed through the Index in 1616.

Not really.  Among the SVs, they do have one of the more balanced views on the extent of infallibility.  In fact, most SVs exaggerate the scope of infallibility because they feel that their SVism (vs. R&R) depends on it.  I do hold the 1616 decree to be infallible, based on the weight of papal approbation.  Theologians argue (I believe correctly) that Holy Office Decrees are not OF THEIR OWN WEIGHT infallible ... unless there's some papal approbation that backs it up as such.  Yes, all decrees of the Holy Office are implicitly approved and endorsed by the pope, but there can be degrees of their endorsement too.  They could agree with it generally or they can agree with it using the full weight of their authority.  Pope can think, "Seems right." when endorsing it or could have a much stronger view.