Changes in the Mass between St Pius V and 1962?

So far as I am aware, that was a charismatic gift of the early Church, a kind of miraculous Google Translate, if you will.  Obviously we don't have that today.

Using the reasoning that "you have to be able to understand the Mass in your own language", then that would mean the Holy Ghost left His Church (or at least the Roman Rite) "high and dry" for over 1500 years, from the waning days of the Roman Empire up until 1965-1970.  That's an awfully long time for souls to be left without something that was supposedly so vital to their spiritual well-being.  In the meantime, we had multitudes of saints (and not just the canonized ones), not all of whom knew Latin well enough to understand the entire Mass.

To suggest that the liturgy of the Roman Church was deficient in its ability to bring souls to greater holiness for 1500 years is sounding a whole lot like "Protestantism through the back door".  They typically skip over the period ca. 500-1500 as "the Dark Ages", when, in their view, Christians labored under a gravely deficient, and even heretical, religion.  Some Baptists even propose an alternate time line during which "true Christians" constituted a non-Roman, persecuted underground Church, a "trail of blood", while apostate Rome (as they see it) melded paganism with the remnants of the Roman Empire and laid a veneer of idolatrous quasi-Christianity over top of it, to which they forced everyone to bow.

And I was not implying that you suggested that.  I regret any such inference being drawn.  My comments were for general consumption, as well as to refute the implicit notion, if it exists, that the Church was deficient in her liturgy for 1500 years.  If it was always the Mind of the Holy Ghost for the faithful to be able to understand every word of the liturgy as well as they understood their own vernacular languages, then that would leave a 1500-year gap where He allowed the faithful to be spiritually malnourished.  People did not all of a sudden quit understanding liturgical Latin in the 19th or 20th centuries.  And Pope St Pius X was very clear that "active participation" referred to music, not to being able to "dialogue" the Mass.  

No, I am not suggesting that the Holy Ghost left the Church after Vatican II.  I have no objection in principle to a vernacular liturgy.  It has precedent in certain missionary efforts (Croatia, China, Native American civilizations) and, truth be told, due largely to Protestant influence in societies such as the United States, there is an implicit expectation that the Catholic faithful will be able to understand everything that happens on the altar.

I will also acknowledge that something had to be done --- via good catechesis or even use of the vernacular in some cases --- to educate people in what was happening at Mass, and not merely to have them be passive observers of an action between the priest and the acolytes.  All that a lot of people understood, was that there was some great mystery going on, and that they'd been taught from childhood that this was something at which they were obliged to be present.  I have heard anecdotes of men going out to smoke a cigarette during Mass, or in Quebec, even smoking in church.  For some, it was a case of "show up at Mass or go to hell".  That was a very flawed notion of what Mass is.

The Church's missionary efforts, as you note, went beyond Mass itself.  Of course the people had to be taught in their own languages, yet Mass is not primarily a teaching tool --- it's not "Sunday school" --- rather, it is the priest offering the Sacrifice of Calvary for the living and the dead (pro vivis atque defunctis) and confecting the Body and Blood of Our Lord.  It's far more than just "presiding over the assembly of the faithful" --- a Protestant praise and worship service does as much.  Do the faithful generally understand the importance of this?

I don't think I'm advancing such a "Mass X over Mass Y" argument. If my comments can be interpreted that way, it's not my intent.

I do advocate greater knowledge of what the Mass is about, and this might be better had through good catechesis outside of Mass. When a large percentage of Catholics do not believe in the Real Presence, do not understand why immodest dress and side conversations aren't appropriate for Mass, do not understand their Sunday obligation, do not understand why they must not receive communion in mortal sin (assuming they even know what that is), and do not understand why they must be attentive not to desecrate even the smallest fragment if they choose to receive communion in the hand, then it's obvious that they're not learning this even at a reformed vernacular Mass. Why not?

FWIW, none of these things are a problem at the TLM.

And I do understand that there is a stripe of Catholic, whose sincerity and holiness I do not question, who would feel cheated if their Mass didn't include a fulsome sign of peace, holding hands for the Our Father recited in union, and being able to raise their voices in song to the tune of "On Angels' Wings" and "Here I Am Lord".

If these things do happen, they are very, very rare.  I can't say that some poorly catechized "seeker" never, ever comes to the TLM with an imperfect understanding of the Real Presence.  You do have the occasional woman who comes to Mass in skin-tight jeans or a low-cut top, but it is very rare, and people who engage in "side conversations" are incredibly rare as well --- in both instances, it would be someone utterly unfamiliar with what is expected at the TLM.  People with any familiarity with the TLM understand the Sunday obligation, and understand what mortal sin is, and why it makes one unworthy to receive Holy Communion.  And CITH is not given at the TLM.  As to the things other than CITH (which isn't an issue because it isn't done), if someone comes to the TLM, returns, and makes it their Mass of choice, they will pick up on the other things in short order.  That is how I can assert that "none of these things are a problem at the TLM".  Such things are self-correcting.

I do maintain that the TLM is more explicit, and more repetitive, as to being the holy Sacrifice, and is more "vertical" towards Almighty God, as opposed to being a more "horizontal" "assembly of the faithful".  The Mass as Sacrifice is still there, the Real Presence is still there, but if you will read the prayers of the TLM and the Novus Ordo side by side, the difference should be clear.  I am not saying that a truncated, simplified Mass, in and of itself, is "lacking" in anything essential to the Faith, and that, in itself, it has been "what has contributed to the decline of the Church", but its implementation has been flawed, and the irreverence of many Masses, as well as a decreased respect for the Sacred Species, is too well-known to bear repeating here.  

Given the massive changes in society, what things would have been like, if there had been the TLM and only the TLM, entirely in Latin, with the familiar rubrics, from 1965 to the present, is a hypothetical that could take many different forms.  One scenario might be what happened in Quebec with the revolution tranquille --- all of a sudden, people just quit going to Mass, for reasons largely unrelated to this liturgy or that, due to Quebec's troubled history WRT the Church, it was a case of "we're not doing this anymore".  The same thing might have happened elsewhere for different reasons.  There's no way to say.  But there most certainly is a "way to say" that availability of the TLM is a case of "supply creates its own demand", and where it goes, it grows.  To try and squelch this, as TC has done, smacks of the recent poor business decisions of Elon Musk WRT Twitter.  He's driving the company into the ground by being so infatuated with his own vision that he can't see what effect it is having --- or, alternatively, he may just not care.  "My way or the highway" is a deeply disliked mindset in today's world, and Elon has very few cheerleaders.