" I guess you would have to believe that Paul was legit in order to believe his writings, and therefore believe the “unforgivable sin” exists "
And you would have to believe he was legit to accept HIS version of the G.N as being the definitive one. He introduces elements to his version of the "Gospel" that we do not find in the purported words of Jesus. But , Paul wrote before any of the 4 Gospels were written, and we have no huge dispute with what he says, even in Acts, where he meets with the Church in Jerusalem, including James.
So I guess it is legit for Christians to accept the writings of Paul. For me he is the Founder of what we call Christianity today, it is his Theology that dominates the thinking of all the Christian sects. Whether this accords with the actual teaching of Jesus we will never know, we do not have any reliable record, that we can be in any way sure of, of what Jesus actually said !