The Seder will be on the 18th as for corporate worship well there will be a Pastor and about 60 people there. That could be considered corporate worship even if not everyone is a member of the church.
I really don't see how this could be considered "corporate worship" since the elements of CP are missing; public pure preaching of the gospel, right administration of the sacraments and church discipline. However, since the Seder was celebrated by alleged believers
how is it that they are allowing unbelievers to take part in it? The Lord's Supper is restricted to those who are professing members of the Church and who are able and have examined themselves before participating.
Number 3 is the sticking point for me. I know it is popular among some "evangelicals" to do Messianic Seders so to get in touch with their Old Testament side and experience the Jewish "roots". But I don't see the point.... however I do think there might be a streak of Theonomy in this but I can't say for sure.
Actually, when I read your post, the first thing that popped into my pea brain was "EMERGENT CHURCH". Such practices are very popular in emergent church circles; going back to the ancient ways, but without understanding the progressive revelation aspect of God's revealed will in Scripture. Types and shadows apparently have no impression upon them and thus the antitypes (fulfillment) of them means little as well. What it all comes down to, as far as I'm concerned, is a lack of recognition of the authority of Scripture and thus of God Himself, the author of it. Personally, I've not heard of any Theonomists celebrating the Seder, but my lack of association with them is hardly any basis for saying they don't do so. Addendum:
I should have added, but I was in a rush when I wrote the above, that there is another element which should be considered; the Regulative Principle of Worship. Although the RP is in regard to the matter of corporate worship, it still has and should have some influence on the worship of God, both ecclesiastically (within the Church) and personal. The foundational principle is that nothing is to be done in the worship of God which is not specifically commanded in Scripture or by example. Adding to or removing that which God has willed in regard to His worship is a serious matter and amounts to idolatry, i.e., 'will worship'; that which man deems is acceptable vs. what God deems acceptable.
You certainly have no warrant to charge this pastor with "falling into Judaism", but if I were you, I would definitely ask him to explain his motivation and warrant to do such a thing.Inquiring minds wanna know!