Order of the Seder
Table set for the Passover Seder
Ur'chatz (wash hands)
Yachatz (breaking of the middle matzah)
Magid (The telling)
Ha Lachma Anya (invitation to the Seder)
Mah Nishtanah (The Four Questions)
The Four Sons
"Go and learn"
Kos Sheini (Second Cup of Wine)
Rohtzah (ritual washing of hands)
Motzi Matzo (blessings over the matzot)
Shulchan Orech (the meal)
Tzafun (eating of the afikoman)
Bareich (Grace after Meals)
Kos Shlishi (the Third Cup of Wine)
Note: The Third Cup is customarily poured before the Grace after Meals is recited...
Eliyahu ha-Navi (cup of Elijah the Prophet)
Hallel (songs of praise) - 4th Cup is consumed after the Hallel
Okay, so it does look like to me that Our Lord is continuing the Seder... Do some look at Our Lord's Cup as being 'added' to the whole cup count? Are there four cups or just three?
It's important to keep in mind how the Passover Seder within Judaism is celebrated today is not necessarily how Passover Seder was celebrated at the time of Christ. There really isn't much documented information about the "order" of the Seder until Talmudic times, and in fact what is documented from 2nd temple Judaism often times seems to contradict Rabbinic Judaism's celebration of it. By that I mean the "order", liturgically speaking, not it's over all theme, tradition, or meaning behind it which has remained the same. But the details. So trying to draw theology by looking at Jesus celebrating a Seder, then trying to plug that meal into a 5th century model is a horrible, horrible mistake, historically speaking.
So while Rabbinic Judaism gives us a very good starting point, it's a big, big mistake to assume 5th century Judaism was the same thing as the Judaism of Jesus day, particularly in practice.
Just try and keep that in mind. It's also important to keep in mind that there was no "one" way of celebrating a Seder in Jesus' time. Much of it was pretty free or so I've read. (see: In the Shadow of the Temple by Oskar Skaursaune)
Judaism was not a monolithic block, and there is in fact archaeological evidence suggesting Judaism used multiple calendars in the 1st century, so Passover could have fallen for different groups of Jews on different days. (usually within one or two days difference) I think I had once read the Essenes used a different calendar (solar? . . .maybe Ialmisry can give some insight on that?) to determine passover, or at least that is scholarly speculation.
The truth is this is not a new debate, and goes way, way back to the early Church. it's been discussed by scholars, ancient, and modern, historians, secular and religious, and everyone in between. And I think the true answer to "was the last supper a Seder?" is we really do not know.
As for Protestant opinions, some of the best NT scholars in the field are Protestant, and many of them say "we don't know"....as do many preachers, and ministers within Protestantism. When I was a protestant I was taught both sides at different stages. In some circles it's strongly favored that it was NOT a Passover Seder, and that Jesus died while the Passover lambs were being slaughtered, thus Jesus IS the Passover lamb. the imagery is obviously powerful, and of course is basically the position of the Eastern Church. But other Protestants believe it indeed was a Passover Seder, which invokes it's own imagery. However i've always found that idea odd, that on the first day of Passover Jews were out trying to get Jesus crucified, and in fact were watching Him and taunting Him, in the first day of Passover. That never made much sense to me. But that's my personal bias.
I guess you should ask yourself what you are seeking. Which is more historically accurate? (ie: what happened in real history) Or are you asking what you "want" to be true based on long standing traditions of your particular Church? The historical question, most scholars contend has no definitive answer as of yet.
Here's a brief article from a Rabbi who contends that it was in fact, NOT a Seder:http://www.interfaithfamily.com/holidays/passover_and_easter/Was_the_Last_Supper_a_Passover_Seder.shtml
There is so much conflicting evidence on this subject in the Gospels, and on many subjects in fact, that since very early on, Christians have always tried to "harmonize" the Gospels, and it always comes up short. This shouldn't destroy anyone's faith, anymore than finding out Matthew the apostle probably didn't write the Gospel of Matthew.....however it still stands firmly in that specific tradition, just like John's gospel does. Maybe some day they'll dig up more evidence to give definitive evidence but historically speaking I don't think there is any way to know for sure, at least not yet.