The SSPX.org website is featuring a good collection of articles on the Liturgical Movement, with each installment receiving a brief summary here: https://sspx.org/en/news-events/news/50-years-new-mass-ongoing-series-liturgical-movement-vatican-ii-54668

I am particularly interested in the subject matter broached in Part 7, which introduces the reforms of Pope St. Pius X, and which is when, historically, things get a little confusing, particularly in understanding St. Pius X's intentions (Part 7: https://fsspx.news/en/news-events/news/50-years-new-mass-saint-pius-x-and-liturgical-movement-7-54488).

Specifically, the matter of whether St. Pius X ever actually called for "active participation."

From the article:

"Intervention Become Necessary
Faced with this observation, Saint Pius X considers “Our first duty, without further delay, to raise Our voice at once in reproof and condemnation of all that is seen to be out of harmony with the right rule… in the functions of public worship and in the performance of the ecclesiastical offices.”
He then adds a capital sentence which will play a decisive role in the evolution of the Liturgical Movement: “Filled as We are with a most ardent desire to see the true Christian spirit flourish in every respect and be preserved by all the faithful, We deem it necessary to provide before anything else for the sanctity and dignity of the temple, in which the faithful assemble for no other object than that of acquiring this spirit from its foremost and indispensable font, which is the active participation in the most holy mysteries and in the public and solemn prayer of the Church.”
A remark is essential here. The expression “active participation”—the partecipazione attiva in the original Italian—will be repeated and will become a leitmotif of the late liturgical movement.
The expression is thus found eleven times in Vatican II’s constitution on the liturgy, Sacrosanctum concilium. It will serve to justify the most daring innovations and the post-conciliar liturgical upheaval. It is obvious that this new concept of the participation of the faithful is a profound distortion of the Holy Pope’s thoughts. Besides, the Latin text of St. Pius X’s motu proprio says in this place: “participatio divinorum mysteriorum” (“participation in the divine mysteries”), whereas Vatican II speaks of participatio actuosa (“active participation,” repeated in numbers 14, 19, 26, 27, 30, 41, 50, 79, 114, 121, 124)."

There are some questions which arise here, for which I have been trying to get clarification for a couple years, without being able to reach any definitive conclusions supported by primary docuмents:

Dr. Carol Byrne says that, contrary to custom, this motu proprio was originally composed in the vernacular (Italian), and that this original version DOES contain the Italian phrase "active participation," (but suspiciously so, and without certainty that it was inserted by St. Pius X), but when (much) later the official Latin version was published, this phrase is absent:

"Some points of concern

The motu proprio was first published in Italian on November 22, 1903, in the Acta Sanctae Sedis, the official organ of the Holy See, but the Latin version bearing the same date did not see the light of day until much later, after many intervening docuмents. Both texts can be accessed .pdf]here. (4)

Pope Pius X's aim was to reform Gregorian chant, not active participation

This wide separation of the texts is a departure from the protocol observed by the compilers of the Acta Sanctae Sedis, who normally published vernacular and Latin texts consecutively for the purposes of transparency and convenient reference. Furthermore, it was uncharacteristic of the Holy See’s policy to issue a legislative docuмent of such weight and solemnity concerning the entire Catholic world in the vernacular and only much later in the universal language of the Church.

Another notable anomaly is the manner in which the Latin version is dated. Instead of the customary format found in the Acta Sanctae Sedis since 1865, it was written according to the method of calculation of the ancient Romans as X Kalendas Decembris. Thus the impression is given that the Latin text had been composed long after TLS, as if it were an afterthought and of relative unimportance. Only those who are familiar with the ancient dating system would realize that X Kalendas Decembris is, in fact, the equivalent of November 22, the same date as TLS. (5)

This has prompted some to assume that the Italian version, simply because it appeared first, is the official papal text. (6) TLS may be “official” in the sense of having been published by officials of the Vatican bureaucracy, but the fact remains that the Latin is invariably the only authoritative and official version of papal docuмents, even if it happens that this text only becomes available later.

Out of sight, out of mind

Therefore, it is to be deplored that the Latin version was buried from immediate view and relegated to an inconvenient position. To add to the difficulties in locating the Latin text, the page number in the Acta Sanctae Sedis was printed as 587 instead of 387, thus misdirecting the researcher.

Why such obfuscation surrounding the only version of the motu proprio (i.e. the Latin) that conveys in indisputable terms the mind of the Pope? The answer will become clear when we come to examine the important discrepancies between the two docuмents.

Which version to follow – the Italian or Latin?

As the use of Latin in drafting docuмents was considered by the Church as the ultimate safeguard of objectivity, it is vitally important for the faithful transmission of the truth in a seamless way. Later generations of Catholics can recognize in the Latin words the exact meaning intended by the Popes. Thus it averted the risk of misleading the faithful through imprecise formulae or the rapid changes in meaning typical of vernacular languages.

As we shall see, misrepresentation is exactly what happened when TLS was placed into the hands of liturgical reformers. An examination of this docuмent will show that it contains a number of key words and phrases for which there is no translational equivalence in the Latin version.

In other words, ideas had been inserted into TLS that pander to the aims and objectives of those who wanted to change the liturgy in ways not envisaged by Pope Pius X. Someone even managed to get the word attiva (“active”) written into the text of TLS to describe the participation of the laity, a term entirely missing in the Latin version."
https://www.traditioninaction.org/HotTopics/f073_Dialogue_1.htm

So, the ultimate question here is: Did St. Pius X ever actually insert the word "attiva" into the Italian (non-authoritative) original?  If not, who did it, and why?  Why the difference in procedure in the publication of the motu proprio?

Was the hijacking of the liturgy already taking place during the pontificate of St. Pius X?

It would seem that the liturgical movement (already modernist by 1920) preferred to follow the Italian (original, but unofficial, and possibly falsified) version, which included the phrase "active participation" and hence gave credibility to the invention of the dialogue Mass (already under St. Pius X's immediate successor, Benedict XV), and from that innovation, the movement never returned to Catholic principles (later overthrowing the traditional Holy Week rites by reforms between 1951-1955, tampering wiwth the canon of the Mass, and eventually bringing these false principles to their logical fruition in the Novus Ordo Missae).