"In Christ"?
It depends on just what you mean by the phrase. If you mean it in the sense that all believers are therefore "in Christ" then fine, even Peter used the phrase in that sense, but if you are referring to the identification truths spoken of by Paul (Romans 6-8 and elsewhere) then, no. Converts of the Twelve were members of the Kingdom of Israel and where not members of the Body of Christ.
As for whether they heard Paul's gospel, I have no doubt that some did and some did not. It wouldn't have mattered if they had or not because God's callings are irrevocable (Rom. 11:29) or, as Paul put it elsewhere...
I Cor. 7:17 But as God has distributed to each one, as the Lord has called each one, so let him walk. And so I ordain in all the churches. 18 Was anyone called while circumcised? Let him not become uncircumcised. Was anyone called while uncircumcised? Let him not be circumcised.
In other words, someone who accepted Peter's gospel and became a believer under the previous dispensation could not become a member of the Body of Christ. Peter, James and John could not stop being Apostles of the previous gospel and become a follower of Paul under his new gospel of grace. It just doesn't work that way. Once you came to Christ under one "oikonomia" then that's where you remained.
Same thing Jesus did. Trust God, obey His commandments. Trust and obey the law. That is the gospel according to Jesus and the Twelve.
The difference, starting with Acts 2 is that there was no longer any place for animal sacrifice or other ritualistic practices that had to do with a High Priest and the Holy of Holies, et. al.
If you want to know specifically what was preached, read the James, I & II Peter, I, II & III John, Jude and Revelation. In short read the New Testament minus the stuff written by Paul.
People, in modern times, who do that and practice what they read consistently call themselves Messianic Jews, whether they are ethnically Jewish or not.
Now, eventually, the twelve were no longer actively evangelical. They were commissioned by Christ to go unto all the world preaching the gospel which He had taught them (Matt 28). There's a very good reason that they agreed with Paul not to do that and instead remain in Jerusalem ministering to the believers there while Paul went to the rest of the world (Gal. 2). That reason being that is had been made clear to them that Jesus was not coming back soon as they had been told and as they had been preaching (Acts 3-4). The Apostles and their converts had sold all of their possessions and lived communally in expectation that Jesus would return and set up Israel's Kingdom. When that turned out not to be the case, they ended up in poverty (as long term communism always does) and were in need of charity which Paul's converts provided and which the Twelve oversaw.
Out of time!
Resting in Him,
Clete