2) The entire Church including the Apostles accepted Paul's writings as inspired and scripture.
3) Paul's writings agree with and is consistent with other scriptural teachings.
4) Paul gave evidence that he was truthful based upon miracles he performed.
The Church, for example, does not teach Pauline doctrine of atonement; the Protestants do.
+Paul was needed to save the Church from extinction. That is not an Orthodox doctrine; it is my view.
The "agreement" among the Apostles is not so clear if you read +Paul's version, rather than +Luke's version. The followers of +James never reconciled with giving up Jewishness.
+Paul created a new religion that ceased to be Judaism. The Church doesn't teach that either; but only someone in complete denial could claim otherwise. Christianity is not Judaism. But Christianity was Judaism until +Paul.
By the time the Gospels and the rest of the Christian canon were written (after 65 AD), the Church was clearly on its way out of Israel and seeking to survive in diaspora in the pagan world.
Judaizers became a liability for the Church's survival. It was either accept +Pauline version of Christianity or perish. The Church in Jerusalem refused yo accept and perished in 69 AD.