If you read any study Bible, like say the new SBL, it will always tell you, the author of John purposefully doesn't have a Seder or Passover. Instead the events of Passion are changed for theological purposes to Nisan 14 in order for Jesus to die at 3 pm, the hour that slaughtering of the Passover lambs begins in the Temple by the priests.
In John's gospel there is no Seder, only the washing of the feet and the discourse. There is a meal, and the anti-communion but no Passover. In John's gospel Jesus is "the lamb that takes away the sin of the world." John's gospel is a "signs" gospel, not a narrative.
Mark is the first or one of the first Gospels written. (According to the SBL the Mark-first theory is now in jeopardy and the traditional view that Matthew is first now seems promising.) All of the texts come from a Jewish and for the Jewish Christian community.
The text about "making all foods clean" at Mark 7:19 cannot be about relaxing kashrut (kosher) laws because when Peter, after Pentecost, receives his famous rooftop vision to "slaughter and eat," he tells God he has never eaten anything that wasn't kosher. (Acts 10) Later in Acts 21, Paul visits the Jewish Christians and gets arrested for demonstrating that he lives the Law of Moses just like other Jewish Christians.
The traditional take is that Mark is the secretary of Peter, and that the Gospel is Peter's take on the matter. I think that is more of a tradition. But, I think it is safe to assume that the text is not talking about the laws of kashrut, otherwise Peter and these Jewish Christians would have stopped observing these laws, like the bishop of Jerusalem, James whose representatives observed kosher and caused the problem that led to the division between Paul and Peter.
Mark's words are about whether it was necessary to visit the mikvah after each time one went to the market. The Pharisees said yes, since priests washed after handling sacrifices. The Pharisees also washed their foods and any bags and vessels they carried they products in.
Jesus was teaching that since the markets already sold kosher food (as they were within the walks of Jerusalem) then all foods were clean. Nobody needed to visit a mikvah (a ceremony bath) to be made clean either.
As for the Passover Seder in the other Gospels, they occur on the 15th of Nisan. This is why Jehovah's Witnesses have a problem dating the Memorial. Christians read the Passion account from John during Lent, and Pastor Russell thought this was literal. He dated the death of Jesus based on the Lenten readings used by the very churches he condemned.
I suggest it might be advantageous for you to take a college course in theology as well as Biblical history. I think you would enjoy both.
You post interesting things. I think you would make a great instructor. You should think about teaching on the academic level.