First of all, you ask one of my favorite questions. Why?
I understand what you are saying about Peter and what the others believed regarding how soon Christ was going to return.
It’s is what I have believed also, and as you say, the sermon in Acts supports that.
I do not claim that Peter immediately knew everything or that he did not develop in His understanding of the message as time went on. I think we can agree that would be wrong.
It is very possible that Paul came along with his revelations and clarified or opened the others eyes to the message of grace.
But, I do not believe that the he was given a different gospel. Remember, Jesus promised that the Holy Spirit would come and lead them to all truth and remind them of what He had taught them.
Paul may have been how the Holy Spirit came to the others to lead them where Jesus wanted them to go in forming the message. We know that many Jews did not accept the message and the Bible prophecies that as well as the fact that the message would be delivered to the Gentiles because of their rejection.
But, a different Gospel? I don’t agree with that. A refining of the message? That I could see.
Catholics are always amazed when converts come into the Church. They bring fresh eyes and a zeal and love for Jesus and the Church.
It is not that cradle Catholics do not love Jesus or the Church. It is more a taking things for granted kind of attitude. And so new Catholics can remind us of the wonders and riches of our faith.
Second, God uses DISTINCTIVE terms to designate the various items of good news: "the gospel of the kingdom" (Matt. 9:35), "the gospel of the grace of God" (Acts 20:24),"the gospel of the circumcision" (Gal. 2:7), "the gospel of the uncircumcision" (Gal. 2:7), "my gospel" (Rom. 2:16), etc. Surely if God DISTINGUISHES between these gospels they cannot be exactly the same.
God has revealed His good news to man progressively. From Adam, to Abraham, and all down through the OT Scriptures we find God proclaiming more and more good news to man. Finally the Lord sent His apostles to proclaim "the gospel of the kingdom" (Luke 9:1-6), but pay careful attention, at that time they DID NOT EVEN KNOW THAT CHRIST WAS TO DIE. With this in mind, read Luke 18:31-34.
After the apostles had been preaching "the gospel" for maybe two years or more, they did NOT HAVE the SLIGHTEST IDEA what the Lord was talking about when He predicted His death. Obviously then, "the gospel" which they preached was NOT "the gospel" which Paul later preached. Or "the gospel" by which we are saved. (1 Cor. 15:1-4).
"The gospel" which Peter and the 11 preached was "the gospel of the kingdom" (Matt. 9:35, Luke 9:2), not "the preaching of the cross".(1 Cor. 1:18).
Just a few examples to mull over..BTW: I love WHY?, too. It leads to study and study leads to learning.