[Inziladun]

Ultimately, I don't think Saruman could have been trusted. He was jealous of Gandalf as things already stood, and he wouldn't have liked being in a position of having to atone for his past evils under Gandalf's watchful eye. Gandalf had taken over as the main mover against Sauron, and as Gandalf noted, Saruman was disinclined to be subservient to anyone. If he had come out and renounced his past actions, I doubt he could have kept his desire to undermine Gandalf at bay indefinitely. He was too far fallen, and I think he would have feigned, perhaps even to himself at first, that he wished to aid the West. But the desire for power and hatred of Gandalf would, in the end, have likely led either to his attempting to overtake Frodo and get the Ring, or informing Sauron of the plan.