On the contrary, the Gospel writers appear to show excellent knowledge about Palestine. This is especially true of Luke, a second generation believer and follower of Paul. Though Greek, he goes out of his way to be geographically and historically correct. Matthew, who like Luke was reliant on Mark (second Generation follower of Paul), clearly demonstrates an understanding of Palestine as well as the Jewish faith.
The fact that Matthew and Luke rely on Mark (as well as Q) indicates an early circulation of gospel writings. By the time of Ireneaus the 4-fold Gospel was so strongly accepted in the Church that he could hold them up on the same level as the four winds, the four corners of the earth, etc. Yet, it's also clear that there was enough question about using the 4 that he felt the need to make the argument. It is important to note that this was very early in church history. Although the gospels were written later than Paul's letters, they were not much later. Certainly within the lifetime of some of the original witnesses (especially if Matthew and John were the Apostles that tradition holds - likewise if Mark was a young disciple of Jesus).
In any case, there is still much debate about the issues we are discussing and not enough certainty among scholars about some of the points you have made. The question of Paul's teachings being alien to the Jerusalem church's views is far from settled.