I happen to attend a chapel that uses the pre-1955 Missal. The bishop has explained the reasons why he does not use the Pius XII Missal with the changes in Holy Week. In short, he contends that these were the first of the Bugnini changes to the Mass and views them has Bugnini's "test run" to see if the world would accept the changes that he had planned for destroying the traditional Latin Rite. He also contends that Pope Pius XII would certainly not have accepted the changes to Holy Week had he truly known the purposes for implementing them.
Personally, I find all these reasons for rejecting the 1955 Holy Week changes to be less than compelling. On the other hand, I can understand the thought process behind the ideas.
I regard this as a debate that is best kept at the episcopal level. The traditional bishops (and priests) should debate this issue amongst themselves and away from the public eye. I would unhesitatingly participate in the Holy Week rites from either the pre or post 1955 Missals according to the determination of the priest at whose Mass I attend. When there is a true pope, the issue will be, at some point, resolved.
On the other hand, if a traditional priest were to declare any other traditional priest anathema because he does not use the particular Missal he uses, I would regard that as schismatic. I have listened to traditional priests from each side of the debate discuss this issue and it seems that it is one issue in which both sides seek to convince the other while not condemning their opponents.
You state the situation well. But because the laity are aware of the difference they naturally seek an answer and Priests, when asked, IMO should give their opinion on it
as their opinion. I don't think anyone can condemn the those who disagree with them on the issue. I wouldn't dare condemn one for obeying the last valid Pope even when considering all the hindsight we have now.
But IMO the pre-1955 liturgy is the safest and least tarnished and I believe I have a right to that opinion as I do not see where such an opinion has been condemned.
But the anti-SVs do indeed use this as a way to divide the SVs though some none-SVs I have no doubt are truly interested in the topic itself. It is not a topic to hang your hat on either side while condemning the other side.
A valid Pope can do what Pius XII.
A valid Pope cannot do what Paul 6 did.
That is the bottom line. When deciding on the plausibility of SV the Holy Week changes should not enter into the debate as nothing was approved that a valid Pope could approve.
Even the changes made by John 23 [apart from inserting Saint Joseph into the canon?] were changes a valid Pope could make. Even the changes make by Paul 6 in 1964 were something a valid Pope could do. But when it comes to messing with the consecration formula an uncrossable line is crossed. We bid such a fool adieu.