Should gentiles be taking part in Passover? They weren't the ones God led out of Egypt. Other days also were celebrated. Passover was an exact rehearsal of the Crucifixion, down to exact times and days of the month in which each occurs - very much unlike the current Christian observance of today. Therefore, a correct observance of Christ's burial and resurrection would overlap Passover precisely. But then THAT would make people realize the inescapable Divinity of each - something the dark forces of this world prefer to to be known.
Christianity is a branch of Judaism. Does not that which affects the root, and whole trunk, not affect the branches as well?
You are exactly correct. That's why all of the ceremonial laws were to point to the coming Messiah. They were to prepare them as a nation to be not only looking for His coming but be a consecrated people to evangelize the world. Imagine the astonishment and disappointment of the angels to see their beloved leader to be born in a manger with only a few in attendance. Kings of all nations should have been there.
Yes, Christianity is a branch of Judaism as much as todays Church leaders distance themselves from it. We are to be grafted to the Vine.