Unfortunately, I don't have the time to write out the responses your questions deserve, so please forgive the brevity and lack of references.
I. The Jerusalem council’s decision regarding circumcision and Christians following Jewish custom as seen in Acts 15 set the precedent.
Do you disagree with this passage regarding Gentiles? Are we Gentiles called to become Jewish again?
For it seemed good to the Holy Spirit, and to us, to lay upon you no greater burden than these necessary things: that you abstain from things offered to idols, from blood, from things strangled, and from sexual immorality. If you keep yourselves from these, you will do well.
No, Gentiles are not required to become Jews. (Just as Jews should not be required to become Gentiles - I know that's another debate, but this is how 'the Church" has treated the Jews the past 1900 years.)
However, there is no indication the original congregations (where the Gentile-believers came from the group of "God-fearers" who sat in on the Synagogue meetings) did not
honor such things as gathering on the Jewish Sabbath, celebrating the Feasts of the Lord, . . . basically worshiping God as the Jews did.
II. Paul addressed this in the letter to the Galatians… Was his labor in vain to teach them the Gospel he wondered?
10 You observe days and months and seasons and years. 11 I am afraid for you, lest I have labored for you in vain.
There is a difference between the obligation to the Law and that of free-will celebration.
I've participated in a few Passover Seders, and the parallels of the ceremony elements to the sacrifice/death of Jesus and hope for His resurrection and return are quite remarkable - even the traditions that had developed over the years (i.e. the Hidden Matza).
It was also an eye-opener to discover that "Pentecost" is the Greek word used for the Jewish Feast of Weeks, the feast celebrating the Giving of the Law. The parallel being that 50 days after Jesus rose, the New Law was given.
(The prophecy of writing the Law on our hearts.)
Then it was pointed out to me how Jesus died at the same time the the Passover Lamb was sacrificed at the Temple.
And Jesus rose from the dead on the day the Jews were celebrating the Feast of First Fruits.
Can you see that these Feasts were designed by the Lord for His people to join into His Passion?
III. Paul also addressed this in the letter to the Colossians:
16 So let no one judge you in food or in drink, or regarding a festival or a new moon or Sabbaths, 17 which are a shadow of things to come, but the substance is of Christ.
Yes, a shadow of things to come. most Christians don't understand how exactly they are a "shadow of things to come" though. (See my above point.)
Now compare how the Feasts prophecy the death and resurrection of Messiah and the outpouring of the Holy Spirit to all flesh with what our traditional Christmas and Easter celebrations symbolism do or do not or reflect.
I say, having done both, there is a difference between becoming an active participant in the remembrance of Passover and that of sitting in a pew listening to a sermon or watching/listening to an artistic demonstration and singing a few songs.IV. Before we go there when Paul called on the Corinthians to "keep the Passover" in 1 Corinthians 5, he was clearly speaking of this in spiritual terms. Do you agree or disagree?
I believe it to be both.