I think Jason's thoughtful post deserves a thoughtful response, so here is my attempt point by point.
1) It's true that Pat Meletios only called the Council after he knew that the Greek Church under the newly elected Abp Chrysostomos approved of changing the calendar. But as you note later, the synod under Abp Chrysostomos depended a great deal on the patina of legitimacy that Pat Meletios' Council gave to the reform, and indeed it did not take place until after the Council had concluded. As for the Council, since it was so poorly attended, with no official representation from any other Patriarchate, I'm not sure that Pat Meletios' supposed fairmindedness to different opinions counts for much. To all appearances it's only purpose was to be a platform for Pat Meletios to air his own renovationist vision for the Church.
So yes, Pat Meletios was not the direct instigator of the reform in Greece, but he played a crucial role in legitimizing the process for the reformers in the Greek synod, and ultimately it doesn't matter whether one innovator named Meletios or another named Chrysostomos is responsible. What matters is that the spirit behind these reforms was anti-Orthodox.
2) I'm surprised you find nothing objectionable in the desire to celebrate the feasts together with the Western heterodox. The whole problem with that sentiment, as found in the 1920 encyclical by the EP locum tenens, and later found in Pat Meletios' own words, which I quote below, is that it presupposes that the Western heretics are nevertheless part of the Church in some way, so that celebrating together with them furthers the necessary goal of unity. But if the truth is, as the Church teaches, that the Westerners are in no way part of the Church, then celebrating feasts with them does not strengthen the unity of the Church, but rather pushes the Church towards union with heresy.
Pat Meletios wrote to the Church of Greece on Feb 3 1923 that changing the calendar at his forthcoming council was necessary “so as to
further the cause, in this part of the Pan-Christian unity, of the celebration of
the Nativity and Resurrection of Christ on the same day by all those who are
called by the name of the Lord.” But the Westerners are NOT "called by the name of the Lord"! The point about ecumenism is not merely the idea of uniting in communion with those who do not share our faith (although I will show below that in fact this has already happened), but even allowing that not sharing the Orthodox faith does not preclude participating in some way in the grace of the Church.
The two pastoral reasons for calendar change that you mention, such as having the Church calendar match the civil calendar, or making it easier for Orthodox in foreign countries to go to Church on feast days, are I suppose worth mentioning, but even you must allow that such pastoral considerations were not the only ones being put forward by either Constantinople or Greece. In any case, I would personally disagree that having Orthodox celebrate their feasts together with the heterodox, and now fully secularized holidays like Christmas, would aid in preventing apostasy. But even if this were a good reason to change, you can't deny the presence of the ecumenist motivations as well.
The closeness with the Anglicans that you mention does seem to have passed away, although this is presumably more to do with the extreme moral and doctrinal liberalism of the Anglican communion than with a lessening of ecumenist efforts. The Anglicans are so far gone, and this dissolution has occurred so fast, even the most enthusiastic Orthodox ecumenist can't quite pretend that union is worth pursuing. This case is utterly different from e.g. the Roman Catholics, who now enjoy extremely close relations with the EP, as you must be aware.
3) You say what?! Firstly, heretical union has occurred, between the Orthodox in Alexandria with the Copts, and between the Orthodox in Antioch with the Syriac church. The mutual prayer that goes on between the EP and the Vatican (remember 2006? have you even watched the video?) easily matches anything cooked up by Pat Athenagoras. I can only conclude from your third point that you are blind to everything that is going on today in the ecumenical movement. Whether willfully or not, I can't say, of course.
By the way, what is your point each time you bring up the presence of the Anglican bishop at Met Vitaly's consecration? That Met Vitaly approved of ecumenism and therefore we should too? Or isn't the more reasonable interpretation that this was a relatively small slip in the life of a hierarch who was later famous for his vehement opposition to ecumenism, including support for the anathema against ecumenism of 1983?
4) Pat Meletios proceeded with the conference without even any official attendance by other Patriarchates. It doesn't sound like he was really that interested in having them participate. And it doesn't look like the reaction of other churches was as positive as you make out:
"The most important decrees
of the Congress were the decisions to change to the new style [calendar] and
to allow the clergy to marry a second time. The Alexandrian, Antiochian and
Jerusalem Churches did not participate in the Congress, considering its
convening untimely [and Meletius an uncanonical usurper]. But its decrees
were rejected by them as being, according to the expression of the
Alexandrian Patriarch, ‘contrary to the practice, tradition and teaching of our
most Holy Mother Church and presented under the pretext of being slight
modifications, which are probably elicited by the demands of the new dogma
of “Modernism”’ (epistle to the Antiochian Patriarch, 23 June, 1923). The
representatives of the Russian Church Abroad [Archbishops Anastasius and
Alexander], and after them the Council of Bishops, reacted completely
negatively to these reforms."
That was from a history of the event by ROCOR Abp Nicon (Rklitsky), in his biography of Met Anthony (Khrapovitsky).
I think it's reasonable to suppose that Pat Meletios did want pan-Orthodox support (or at least no active opposition) to his reforms, but that doesn't take away from the following facts: his ideas for reform were animated by heretical ecumenism and anti-Orthodox modernism, not love for Orthodox tradition; the other churches' reaction to his council's proposals was overwhelmingly negative, even if a break in communion did not result immediately; whatever the legitimacy of his council's acts, the reform proceeded unilaterally and uncanonically in a small number of individual local churches, as you yourself concede.
After Pat Meletios resigned from the Ecumenical throne, the following Patriarch did indeed wish to consult with the other local churches before agreeing to Abp Chrysostomos' proposed calendar reform. Here is what the Patriarch of Alexandria had to say:
that, without any real cause or dogmatic or canonical reasons, the brotherly
advice and entreaties of the four Apostolic Thrones has been rejected, and the
‘reform of the calendar’ has taken place, caused us great grief and surprise.
You are in danger of alienating all the Orthodox peoples of the Church.
Therefore I suggest the convening of a council to examine the question.
Taking into consideration the letters from the Churches of Romania and
Serbia, we abide in these things which have been dogmatized in former
Synodal Congresses, and we reject every addition or any change of the
calendar before the convocation of an Ecumenical Council, which alone is
capable of discussing this question, concerning which Ecumenical Council we
propose a speedy convocation."
This, of course, written on Jan 15 1924, shows that the other Patriarchates did not consider the 1923 congress to have represented or accounted for their opinions, or to have had pan-Orthodox authority.
5) Personally I think the jury's still out on the legitimacy of the 1583 sigillion. As I pointed out before, Met Cyprian's article does not represent new groundbreaking research, but only a regurgitation of old New Calendarist polemics, which were already alluded to in the letter of HTM monastery I cited above. Basically I would not offer an opinion about the authenticity of the sigillion without also reading what Old Calendarists argued in response to the forgery claims back in the 1920s and 1930s. Overall I think it's suspicious that no one claimed forgery before the reform, even Abp Chrysostom when he offered his opinion on the consequences of unilateral calendar reform. Wouldn't that have been the perfect opportunity to point out that the Church had never condemned the Western calendar, or at least the Western menologion?
Given that the 1923 council was not interested in preserving tradition, is it really that surprising they would neglect to mention previous Church decisions that forbad any alteration of the calendar to align with the Western heretics?
6) If the other Local Churches were to view the Church of Greece as schismatic, that means that Abp Chrysostom understood that the Church's voice on the matter of unilaterally changing the calendar was unambiguous enough that other churches could not fail to see that the Greek Church was schismatic. To me Abp Chrysostom's words at the time show that he well understood what the Church had decided.
The opinion of other local churches on whether one church has fallen into schism is valuable, but not essential. For instance, what do you say to the fact that your own ROCOR recognized the Old Calendarists in Greece as the legitimate Church of Greece, consecrating bishops for them? Why does their opinion not count, while the opinion of Jerusalem in 1937 does count? Since there was obviously disagreement and uncertainty among the several jurisdictions as to the implications of the calendar reform in Greece and elsewhere, we can't simply point to what one or other church has said and rely exclusively on that. We have to consider also the evidence for what the whole church had already said about the Gregorian, not only in 1583, but 1587, 1593, 1722, 1827, 1848, 1895, and 1904, and about the declared motivations behind the reform of 1923-4, which are either ecumenist ("we must celebrate together with the Western Christians") or modernist ("we must conform Church practice to secular usage; tradition is worthless if it's not scientifically up-to-date").