I do not know where you are getting your information on what actually happened at Nicaea. We do not have that many actual primary sources on the council. I have looked at the few sources that we have and they agree with my argument that the 1st Ecumenical Council declared that the Church would follow the Roman practice for the calculation of the date of Pascha.
The council agreed that there should be a uniform date, and that the date should be computed independently, as was already being done in "Rome, in Africa, in all Italy, Egypt, Spain, Gaul, Britain, Libya in all Achaia, and in the dioceses of Asia, of Pontus, and Cilicia." (Constantine, Letter to the absent bishops, in Eusebius, Life of Constantine,
The Romans celebrated Pascha on the Sunday after the Jewish Passover. Therefore the council also mandated that Pascha follow the Jewish Passover.
Yet again, you are being inconsistent in your definition of "Passover." By "Passover" do you mean
(a) the 14th of Nisan as computed on the Jewish calendar?
(b) the 14th of Nisan as computed by Christians? or
(c) the 15th of Nisan as computed on the Jewish calendar?
Sometimes your statements seem to presuppose one of these, then a subsequent statement presupposes a different one. Your two statements just quoted are an example of this. Your first statement is almost correct if one uses definition (b). But your second statement, read in light of your other statements on this thread, clearly presupposes definition (c).
In Alexandria, Easter was set to the Sunday after "Passover" as defined by Christians, definition (b). The date of this "Passover" varied by a 19-year cycle, similar to, but not identical to, the Julian Paschalion of the present day. In Rome, as far as we know, the "Passover" in question was also a Christian Passover, definition (b), whose dates were cyclic in the civil calendar at intervals of 84 years. Also, sometimes the Romans waited for the second Sunday after the Christian Passover. They seemed to think that, since our Lord rose on the 16th of the moon by the Johannine chronology, Easter should not fall before the 16th of the moon. In any case, "Passover" definition (b) was the one that was implicitly approved by the council. Definition (a) was what Athanasius refers to when he states that "The Syrians, Cilicians, and Mesopotamians...kept Easter with the Jews." (De Synodis
1.5). So definition (a) (and by implication definition (c)) was deprecated by the Council.
The difference was that the council decreed that Orthodox would make their own calculations instead of relying on the Jewish calculations for when the 1st full moon falls after the Spring Equinox.
Here you presuppose that "full moon" = "Passover" definition (b) above. But you frequently contradict this statement elsewhere by presupposing that the council commanded that we use a "Passover" defined by definition (c) above.
The council gave the responsibility to prepare the tables indicating the date of Pascha to the Patriarchate of Alexandria. That is why St. Athanasius sent out his Festal Letters...
The council did not micro-manage the Paschalion. In particular it did not give any special privileges to the Bishop of Alexandria. The council seems, rather, to have assumed that the foremost bishops would work out the details in practice through consultation. The sequel shows this process at work. Nowhere in his writings does Athanasius pretend to any special privileges in the Easter computation. On the other hand, Athanasius clearly consulted with Rome on the question, and sometimes when the two systems gave discrepant computations, Alexandria adopted the Roman date. Other times, Rome adopted the Alexandrine date. Easter 346 and 349 seem to be examples of the first, Easter 330, 340, and 341 seem to be examples of the second. In 333 the two cities agreed on a date that was according-to-rule for neither. For 349 the Aramaic index to St. Athanasius's letters gives us a picture of some of the horse-trading:
In this year, Easter-day was on the 30th of Pamenoth, the 19th day of the moon, VII Kal. April (March 26th)]...because the Romans refused [the Alexandrine date of April 23rd], for they said they held a tradition from the Apostle Peter not to pass the twenty-sixth day of Pharmuthi (April 21st), nor the thirtieth of Phamenoth (March 26th).
Furthermore, Athanasius's Festal letters are addressed only to his own bishops, not to the whole world. And he would have sent them even if the council had not considered the Easter question.
Theophilus, a later bishop of Alexandria, sent a 100-year Easter table to Emperor Theodosius. In his cover-letter to the Emperor, he nowhere explicitly cites any special legal privileges of his see, though he seems to be relying on Alexandria's reputation as a city where astronomy is done well. Likewise in his brief description of principles of the Easter computation, he never cites councils or canons. The only laws he cites are those of Scripture, meterology, arithmetic, and astronomy. His underlying presupposition seems to be that the way of distinguishing good Easter tables from less good Easter tables is by how well they uphold the principles of (1) the equinox (2) the full moon and (3) the Lord's day, not by whether they come from Alexandria or not. The idea that Alexandria had some sort of special privilege in the matter arose only later, perhaps with bishop Cyril.
As I mentioned I participated in the North American Lutheran Ecumenical Dialogue. When the Lutherans brought up the Aleppo proposal, the representatives of the Greek Archdiocese cited the response of the Ecumenical Patriarchate to the document. The Greek representatives reported that the consensus within the Eastern Orthodox Church is that Pascha must follow the Jewish Passover. Since the requirement that Pascha follow the Jewish Passover was part of the original calculations of Rome and the 1st Council of Nicaea specified that the Church would follow the Roman standard for the calculation of Pascha it is logical to assume that the 1st Ecumenical Council required that Pascha follow the Jewish Passover.
Here again you switch from "Passover" by definition (b) above to "Passover" by definition (c) above. All the independent Paschal computations in use at the time of the council presupposed definition (b), so definition (b) is the only one that the council can be said implicitly to have ordered. Definition (a) was deprecated, and so it is logical to suppose that definition (c), on which you rely, is also implicitly deprecated by the Council.
There are several problems with the Gregorian Calendar calculations of the date of Pascha. The first is that they overturn the decisions of the 1st Ecumenical Council which also gave the Patriarchate of Alexandria the responsibility for preparing the tables indicating the date of Pascha.
The council did not do this; see above.
The Eastern Orthodox Church follows the council and still uses the tables prepared by the Patriarchate of Alexandria.
By using the tables that reached their final form in the 5th century, you are departing from the intention of the conciliar generation. It is the Gregorian Paschalion that is faithful to the Council, as also Milankovitch's proposal would be, and the Aleppo proposal would be.
The Gregorian calculations not only violate Nicaea 1 by celebrating the Western Easter before the Jewish Passover, it also also allows for the Western Easter to fall on the day of Passover as it did in 1825.
Here again you are changing the definition of "Passover" to agree with your prejudices, rather than using the definition that is implicit in the arithmetic of the Paschalion.