Nicodemus was a Pharisee and a convert and so was Paul. Both were figures of authority. From these examples alone we know that some Pharisees had the true faith of the Prophets and were prepared to accept Christ.
Yes, but they did NEED to convert because what they knew before as Pharisees was wholly incompatible with Christianity. Had they been Moses or Abraham, I wonder if there would have been any need to convert. I can't see either Moses or Abraham, or other OT righteous, as ever having a belief system compatible with being a Pharisee. They would have seen the corruption immediately, I would guess. So, I don't think a "good" and "loyal" Pharisee could have had true faith. Only the "afflicted" ones, like Nicodemus and Paul did.