I question how much Pope Pius XII even endorsed some of the 1955 changes.  He was pretty ill by then and by all accounts the curia were running the Church.  In his letter about the new Holy Week Rites, he mentions very little regarding changes to the rites themselves, but was focusing almost entirely on restoring the proper time of day for some of the Liturgies.

From Wikipedia about Pope Pius XII:

A long illness in late 1954 caused the Pope to consider abdication. Afterwards, changes in his work habit became noticeable. The Pope avoided long ceremonies, canonizations and consistories and displayed hesitancy in personnel matters. He found it increasingly difficult to chastise subordinates and appointees such as his physician, Riccardo Galeazzi-Lisi, who after numerous indiscretions was excluded from Papal service for the last years, but, keeping his title, was able to enter the papal apartments to make photos of the dying Pope, which he sold to French magazines.  Pius underwent three courses of cellular rejuvenation treatment administered by Paul Niehans, the most important in 1954 when Pius was gravely ill. Side-effects of the treatment included hallucinations, from which the Pope suffered in his last years. "These years were also plagued by horrific nightmares. Pacelli's blood-curdling screams could be heard throughout the papal apartments."

Pius XII often elevated young priests as bishops, such as Julius Döpfner (35 years) and Karol Wojtyła (later Pope John Paul II, 38 years), one of his last appointees in 1958.
...
With frequent absences from work, Pope Pius XII had come to depend heavily on a few close colleagues, especially his aide Domenico Tardini, his speechwriter Robert Leiber, and his long-serving housekeeper Sister Pascalina Lehnert.

One might seriously question whether and to what degree the 1955 changes even came from Pope Pius XII, or whether he was of sound mind by then, given toe hallucinations and hesitancy.  At the very least, he was in a state where he could be easily manipulated by certain nefarious Cardinals.  IMO, there's a decent possibility that Gleazzi-Lisi and/or Niehans were deliberately poisoning or debilitating Pius XII.

The interesting thing is that he wrote 10 encyclicals between 1956-1958.  That's quite a heavy workload, and contrasts with the Wiki description, which speaks of noticeable changes in his work habit during these years.

A new question is forming in my mind:

What is the binding nature of papal acts which result from manipulation by subordinates? 

How is the disciplinary infallibility of the Church engaged -or not engaged- in such situations?

In any case, it sounds like Pius XII suffered an horrific death (and what the heck is "cellular rejuvenation treatment?"), and this article increases my eteem for him.

If Pope Pius XI was poisoned, why not Pope Pius XII?

These are good questions, and I don't know the answer.  We could have a scenario where a Pope develops Alzheimer's disease and barely knows what day of the week it is, and some corrupt Cardinals just keep taking subversive stuff to him to sign.  "Here, sign this." ... and he signs anything that's put before him not even knowing what he was signing.  I would guess that these types of things would not be considered legitimate.  I don't know that Pope Pius XII was that far gone, but I could very easily see some level of manipulation going on.  It was +Cicognani who issued the decree on the Holy Week Rites, claiming that the Cardinals had briefed Pius XII in detail about them and that he approved.  What would have stopped him from withholding certain details?  If you look at the decree, there's only a single passing mention of the New Ordinal, and all the content but that phrase was about the change of times for the various Liturgies and extending the fast through midnight on Holy Saturday, none of which I can disagree with.  Pius XII certainly could have read it and agreed to it without knowing exactly what was in the actual ceremonies.  These Cardinals could easily have minimized the changes when presenting them to him ... as just a few details here and there.  And EVEN THEN, the docuмent is not signed by or issued by Pius XII, but by +Cicognani.  To me, this would fall into the same category as decrees of the Holy Office, which are not considered inherently infallible based on some generic implicit approbation by the Pope, but only if the Pope explicitly confirms a decree.

Let us keep in mind that Archbishop Lefebvre, who was appointed Apostolic Delegate to French-speaking Africa by Pope Pius XII in 1948 and remained so until after the death of the Angelic Pastor, was received in audience by him yearly in this capacity and collaborated with him in writing the encyclical Fidei Donum on the missions in 1957. The Archbishop had nothing but praise and esteem for Pius XII and, as we all know, insisted on accepting his liturgical reform. It does not seem credible that the Pope could have been in such a condition that he was not responsible for his actions and Archbishop Lefebvre oblivious to such a state of affairs. 

Thanks for posting, Sean. What a great gift of God is Archbishop Vigano. How is it possible that the SSPX are not standing shoulder-to-shoulder with this extraordinary convert to Tradition? It is iniquitous! "Let your speech be yea, yea; no, no; and that which is over and above these is of evil". But Lord, have you forgotten your virtue of prudence? As BW says, they prefer to be in Huonderland. Nothing has changed! Nothing shows more clearly what a caricature it is of the Archbishop's Society.

How is it possible that the SSPX are not standing shoulder-to-shoulder with this extraordinary convert to Tradition? It is iniquitous!

Declaring yourself an ally with Francis's primary enemy is bad for canonical recognition (or maintaining it, if it has already been given).

This example alone shows the folly of believing one can remain traditional amidst conciliar contagion.

All the once-traditional orders which later capitulated became conciliar attack dogs against those who had not yet capitulated (and this is one of the causes of the SSPX hatred of the Resistance.  There are additional causes.).

I agree with Abp. Viganò, and for me, the following argument suffices to adhere to the rubrics of St. Pius X

1. A disciplinary law of the Church cannot be intrinsically evil, nor, at the time, can it be extrinsically evil.

2. Over time, a disciplinary law can become extrinsically harmful or difficult to observe, given a change in circuмstances. This is universally admitted by theologians.

3. In such scenarios, the law ceases to bind. This is the well-known principle of 'epikeia'. It does, however, require the absence of the lawmaker (which I admit; I adhere to the Thesis of Cassiciacuм).

4. There are numerous elements, Catholic in themselves, in the reformed Holy Week, which now take on a Modernist connotation owing to their inclusion in the New Mass with a Modernist intent, such as the use of vernacular, greater participation of the laity and more. Another example of a pre-Conciliar reform which takes on such a connotation is the dialogue Mass.

5. The principle architects of the Holy Week reforms — Bugnini and others — openly admit that the reforms were part of the whole, which whole ended with the New Mass. Paul VI even promulgated the New Mass as a continuation of the reforms.

6. Invoking the principle of epikeia, then, we should not observe the 1955 Holy Week reforms, for 'he who consents to the beginning consents to the end'. If our goal is the preservation and propagation of Catholic doctrine, discipline and worship, let us preserve that pristine Catholic liturgy untouched by the hand of Bugnini.

Yup.

As a Resistance priest said to me today in a private email, "The Pian Holy Week is dying of natural causes."

Let it be buried soon.