Two issues here. First, I still don't think Saruman had the slighest clue that the Ring wasn't in Rohan. Sure, he may have known that there were only two hobbits where there once were four. However, no one suspected that the other two had split off to enter Mordor, not even the Eye could see that. The logical deduction that would be that either the missing Hobbits had died like Boromir, or had abandoned the quest. With the "major" players in the Fellowship in Rohan, it is obvious that the Ring would be there as well.
As far as Sauron's fear, I doubt he had any regard for Saruman (or Galadriel, for that matter), with or without the Ring. This is fairly evident with the "This dainty..." quote. Sauron expected Saruman to challenge him for mastery of the Ring, it was something he had planned for, and probably had all sorts of contingencies mapped out for if that came to be. Saruman may have thought he was pulling the wool over Sauron's eyes (or eye, heh), but he was not. Sauron was ready for him, and he would not lose that battle.
I think Sauron's primary fear was Gandalf. Gandalf's motives, being completely pure, were completely unfathomable to the Dark Lord, and he could neither control nor predict what the wizard would do.
And remember that the One Ring was an instrument of deception; if a character claims he could vanquish Sauron by wielding it, that doesn't mean the claim has any merit. Heck, even Frodo tries to declare himself the Dark Lord at the end of RotK, I don't think any of us could put much stock in that claim.
|