The exodus from Egypt
By celebrating it and keeping its laws.
There are kosher food companies that are now making gluten-free products Kosher for Passover and all-year-round. However, something being gluten-free has no bearing on whether or not it is Kosher or Kosher for Passover.
By celebrating it every year.
To honor those who gave their lives so you could be free.
Passover is a religious celebration not a national one. Those people that follow Judaism in Japan would celebrate it.
It depends on the brand. However, even brands of real mustard that are certified kosher for Passover are still kitniyot.
During Passover Jews relive the Exodus from Egypt and their birth as a nation, both of which were preparations for receiving the Torah from God at Mount Siani.
Some do and some don't. It's certainly not a traditional component of the Passover commemoration. But for those who do include it, perhaps it's the following passage in the ancient text of the Passover seder that inspires them to include the Holocaust in their recollections. It says "Not only one alone, but in every generation they rise up upon us to destroy us, but the Blessed Holy One rescues us from their hands."
During the passover festival, the Jews remember their exodus from Egypt.
No, Bararabbas was the one that was to be released after the Passover. It was a traditioon that one be set free after the passover. Pilate offered Jesus as the one to be released after the Passover but the people requested Barrabas. As the passover was not complete, he would have still been in jail. See Luke 23:18
Yes that was the custom; on Passover Day one prisoner would have to be released. Pilate wanted Jesus to be free, but the crowd wanted Barabbas free, and he was set free at the cost of Jesus.