This is just a few comments on the interesting topic of whether the Last Supper was the passover. Like most people I grew up on the majority opinion that the Last Supper was the Passover. However I'm quite open to the alternative view that you present.
There are none the less some difficulties that remain on this question. The first is Patristic support. Have you done a survery of what the Fathers have to say on the status of the Last Supper? I found by looking up Aquinas' Contra Errores Greaecorum this quote:
"To this, however, Chrysostom replies, commenting on that very text of John: That they might not be defiled, etc.: “What does this mean, but that they ate the Passover on another day and broke the law in order that they might fulfill the most wicked desire of their soul in the death of Christ; Christ, however, did not transgress Holy Thursday in paschal week, but on that day he ate the Passover.” (Lib. 108, 3-8, from Chrysostom Super Matth. hom. 84, n 2 (PG 58, 754).)"
So just doing a small bit of research that is one significant Father on side (not to mention with Thomas' support).
Also have a look at this homily from Benedict XVI. I'm not suggesting it is authoritative but he offers in there a defense of the Last Supper being the Passover especially based on new knowledge from the Dead Sea Scrolls:
R. Sungenis: Matthew, my position on this, and topics like it, is that which is given by Leo XIII in Providentissimus Deus. Unless the Fathers have thoroughly investigated a topic, and after that are in consensus on the interpretation of it, then we are not obliged to court their opinion. We may use it as a help or even a thought-starter, but not as anything definitive. This would be especially true if only a few Fathers commented on the topic. And, when we find that the few that treated the topic merely make an assertion (as does Chrysostom's above) and do not provide any detailed exegesis of the passage in question, then it really has little value for us. Leo XIII taught that in such cases, we are obligated to go to Scripture and examine the original languages, which is what I did in my essay on the topic. If someone wants to challenge my position, then they would be required to deal with the exegesis of the passage I offer rather than quote a Father here or there that said something different but without any exegesis. In the larger picture, although the Fathers can be very valuable in arriving at truth, Catholics, by and large, need to refrain from proof-texting their way through the Fathers. We also need to remember that the Fathers did not have any special guidance from the Holy Spirit. They have many opinions on many subjects. In a way, we, today, are in a better position to judge the truth in various instances, since we have the whole patristic and medieval eras to consult; we also have the various instances in which the Church has dogmatized a certain belief, and we also have a great knowledge of the original languages which most of the Fathers and medievals did not have, since most only knew Latin. In a way, we are in a much better position to determine whether Jesus and the Apostles celebrated the Passover on Thursday night. At the least, it is an open topic of discussion.