You ask good questions.
Pauls writings and teachings dominate the New Trstament because Paul dominated the post-cross Christian movement. Because of Pauls popularity, the Gentile world heard about Jesus by way of Paul. People hung on Pauls every word! Even Pauls letters of corospondance to the churches became "scripture". Some of the other original apostles parted ways and went on into the world carrying the gospel as they understood it. Paul was sincere! All in! He meant well! But we have to realize that Paul was not Christ! Christianity is largely a gospel about Jesus according to Pauls own conclusions. We've largely lost the original gospel of Jesus' inspiring and triumphant life as a human obedient to Gods will.