The proof is in the prophecy.
How do you prove the "new testament" belongs in the Bible at all? In fact, how do you know to believe in the Bible at all? It has to be authorized by an authority external to itself (G-d) or else it has no more meaning than the koran or the book of mormon.
The proof is in the prophecy.
No it isn't. If it isn't authorized by the Torah, it is illegitimate.
The only reason you believe the "new testament" fulfills prophecy is because it says it does. But what if it's wrong? How do you know the "new testament" is the word of G-d at all and thus to be believed when it claims to be a fulfillment of prophecy?
I could write a religious book and have it claim to be a fulfillment of prophecy, fcol.
Moses and Paul both claimed to get their messages from direct revelations from God. Face to face and mouth to mouth.