In His incarnation ("in the flesh"), He was sent to none but the lost sheep of the house of Israel (Matthew 15:24). We cannot know Him in His capacity of Israel's Messiah (2 Cor 5:16), but only through the revelation of the mystery (Rom 16:25; Eph 3:8-9).
We can understand him in his capacity as Israel's Messiah, for we have been grafted onto Israel and thereby made partakers of the New Covenant, as Paul says in Romans 11. Salvation is from the Jews, as Jesus declares in John 4.
As for the mystery - you are merely reading your own ideas into the passage. Paul does not state that this mystery is some new Gospel in opposition to that given to the disciples. He does say: " the Gentiles are fellow heirs and fellow members of the body, and fellow partakers of the promise in Christ Jesus through the gospel" (Ephesians 3:6). See here - they are fellow heirs, members, and partakers of the promise in Christ Jesus.
He is not introducing a new promise, separate from what Israel received. Rather he is saying that anyone is free to partake of the promises and blessings of Israel - they are free to be grafted onto Israel as fellow heirs and members of the body.
Your idea that something separate from Israel has been introduced is entirely unbiblical and has no backing from any scripture.
Yes, NOW there is no difference! TODAY there is no difference, and thank God for it! Your problem is, up to and past Acts ch. 10, the wall was still up in the minds of Messianic Jews. Peter was reluctant to even darken a dog Gentile's door (Acts 10:28). Believing Jews were telling only other Jews about Messiah (Acts 11:19). Peter and the rest hadn't yet been told there was no longer a difference between Jews and Gentiles...that means they knew nothing, yet, of the Body of Christ. That knowledge came to them through Paul.
There was a wall in the minds of the Jews, but not in the Gospel - for Christ commanded them to go forth and make disciples of all nations, not just Israel. And scripture does not say that Peter learned that Gentiles were to be accepted through Peter; rather - it had always been acceptable for Gentiles to join Israel as proselytes.
Peter tells us in Acts 15 that "in the early days" God had decided that through Peter the gentiles would here the gospel and believe - and that they received the HS just as the jews did. You cannot say that the events in question here occurred only after Paul, or that Paul was the messenger to Peter. By saying this occurred "in the early days" of the church, he is pointing to things that happened quite some time ago - very likely prior to Paul and as early as the Pentecost.
Regardless of whether it was before or after Paul, the scriptures are clear that there isn't a new Gospel for them, no separate promise from Israel. Rather the gentiles are grafted onto Israel and become fellow heirs and partakers of the gifts and promises of Israel.
The core of Paul's "my gospel" is Christ and Him crucified for the sins of the world, without distinction. If you can find Peter preaching that at Pentecost, you'll have a point.
Peter tells us that from the "early days" of the church God had made this known to him (Acts 15). Jesus, of course, commanded them to go to all nations to make disciples.
It was not made known to the disciples before it was revealed to Paul. It was made known to them TO and THROUGH Paul (Gal 1:11-12).
That passage doesn't say what you think it says. try again.
Yes there is, in Galatians 2.
Galatians 2 doesn't say there is a difference in their Gospels - only in their main demographic. Both tried to reach both jew and gentile.
The Body of Christ is grafted onto nothing. It is its own entity, complete in Christ, untied from covenant Israel.
You are being intellectually dishonest here. You know good and well that Paul teaches that the gentiles have been grafted onto Israel, and thereby made fellow heirs and members of the body.