I can't imagine any Jew upset that a Christian wanted to attend a seder or have their own. I welcome it, and know many other Jews that feel the same.
The issue comes in with the group of Messianics that is trying to make the traditions of the seder mean something else. We have been told every year for centuries what the matzoh means; I'm not happy to hear a Messianic group proclaiming that they know what Jewish traditions mean better than centuries of Jews do.
I can see how there would be strong feelings about this; and the only way to counter it would be --- it seems to me --- strengthening the traditional meaning and reinforcing it constantly within your own community.
It seems to me, though, that ALL of Christianity is a reformulation of, or a competing interpretation of, Judaism. It would be hard to find an element that isn't. I was certainly taught (and rightly, I think) to "interpret" the whole body of Hebrew Scriptures "in the light of Christ."
Moreover, much of Protestantism is, it seems to me, a latching onto Catholic things to make them "mean something else." Again, this is so pervasive that to catalogue it adequately would take hundreds of pages.
There's no stopping it, and no use taking offense. The only response is in constantly reinforcing your own defining lines, in higher contrast.
The issue comes in with the group of Messianics that is trying to make the traditions of the seder mean something else. We have been told every year for centuries what the matzoh means; I'm not happy to hear a Messianic group proclaiming that they know what Jewish traditions mean better than centuries of Jews do.
Our church does a Seder led by a Messianic group every year. Let me assure you that the Messianics are not changing the meaning of the Seder. What they do is give a short class before the Seder on how the passover meal points to Christ. After that they do the Seder exactly as the Jews do without anything added or any mention of Jesus what so ever. (All of these Seders may not follow this format, but the ones I have attended do.)
Christians and Messianics have always looked at the old testament, the Seder, the Jewish wedding ceremonies and other Jewish traditions in the context of how they all point to Christ. Being Christians it is impossible for us to see it any other way. We believe that the Jews are God's chosen people and if our faith is to have any legitimacy at all it must therefore be irrevocably tied to that of the Jews. And if Jesus is both man and God then everything must point to Him.