Did Polycarp argue against changing the day of the Passover Seder from 14 Nisan to Sunday?
Yes he did. There was disagreement regarding the Passover Seder, but the two bishops did not break off communion with each other, and Anicetus allowed Polycarp to celebrate the Eucharist in his church. Ponder that for a moment. Anicetus would not have allowed Polycarp to celebrate the Eucharist in his church if Polycarp did not agree that the body and blood of Christ were present in the Eucharist.
See:
According to Irenaeus, during the time his fellow Syrian, Anicetus, was the Bishop of Rome, in the 150s or 160, Polycarp visited Rome to discuss the differences that existed between Asia and Rome “with regard to certain things” and especially about the time of the Easter festivals. Irenaeus said that on certain things the two bishops speedily came to an understanding, while as to the time of Easter, each adhered to his own custom, without breaking off communion with the other. Polycarp followed the eastern practice of celebrating the feast on the 14th of Nisan, the day of the Jewish Passover, regardless of what day of the week it fell on. Anicetus followed the western practice of celebrating the feast on the first Sunday after the first full moon after the Spring equinox (March 21). Pope Anicetusthe Roman sources offering it as a mark of special honorallowed Polycarp to celebrate the Eucharist in his own church.[12]
https://en.wikipedia.org/wiki/Eucharist
Anicetus did not 'allow' Polycarp to do anything. It was Polycarp who had the connections to the actual Apostles which gave him the authority to come to Rome and argue against heresies. Anicetus was not even on the same plane as Polycarp! And this is the sort of specious authority which has buoyed so much heresy by Rome!
Could Anicetus be giving Polycarp the benefit of any doubt.
Polycarp could very well say “the body and blood of the Lord are here in the communion.” And even modern evangelicals will say they are omnipresent, and available to all believers, “in a spiritual way.”