Wow, the "what if" question is always quite impossible to answer. What would you be like with a different set of parents? Or if you were raised in a different country? Or went to a different school? Of course you WOULD be different, but who could possibly say what you'd be like? Impossible.
> Would it have been more driven by the teachings of James, the Just?
Since we know close to nothing about his teachings, his style, and very little about his personality, it's impossible to say. Who would be the driving force behind the NE Patriots if Tom Brady had never been hired? Who can answer that question?
> Do you believe Christianity would exist today without Saul (Paul)
Absolutely. The message of Jesus was explosive and couldn't be contained. In the NT we have what Paul was taught and shown by God. We can rightly assume that if it hadn't been Paul, it would have been someone else. The truth will always come out.
> do you believe the reforms Paul introduced to Christianity were essential for the survival of the faith?
That's a trick question. Paul says he taught what God instructed him to teach. We can rightly assume that if it hadn't been Paul, it would have been someone else. If that's what God wanted to reveal to the world, it didn't matter if He did it through Peter, James, Paul, or the Verizon guy ("Can you hear me now?").