[lathspell]

Do you think he'd have designed the ring to tempt people who were actually capable of using it to destroy him?

No, of course he wouldn't, but it wasn't his design to loose it in the first place. Sauron never thought that he would loose his Ring when he made it. Those thought were his greatest folly. Thinking that he wouldn't loose it, because no one could withstand his army and he himself together. Another one is that he never thought that the Fellowship would eventually try to get into Mordor, instead of using the Ring. He thought Mordor was closed, yet it wasn't. He thought he'd never loose the Ring, yet he did.

When Boromir offers Elrond at the Council to take the Ring as a weapon and use it against the Dark Lord, Elrond answers:

'Alas, no,' said Elrond. 'We cannot use the Ruling Ring. That we now know too well. It belongs to Sauron and was made by him alone, and is altogether evil. Its strenght, Boromir, is too great for anyone to wield at will, save only those who have already a great power of their own. But for them it holds an even deadlier peril. The very desire of it corrupts the heart. Consider Saruman. If any of the Wise should with this Ring overthrow the Lord of Mordor, using his own arts, he would then set himself on Sauron's throne, and yet another Dark Lord would appear. And that is another reason why the Ring should be destroyed: as long as it is in the World it will be a danger even to the Wise. For nothing is evil in the beginning. Eevn Sauron was not so. I fear to take the Ring to hide it. I will not take the Ring to wield it.'
'Nor I,' said Gandalf.

Now, in this quote two things seem to be made clear very well. The first is that the Dark Lord could be overthrown if someone else with a great power of his own should take the Ring and wield it. The second is that he says that he (Elrond) won't take it to wield it, meaning that he was indeed strong enough to wield it. Galadriel's quote when she gets the Ring offered freely from Frodo fits into this as well.

'I do not deny that my heart has greatly desired to ask what you offer. For many long years I had pondered what I might do, should the Great Ring come into my hands, and behold! it was brought within my grasp. The evil that was devised long ago works on in many, whether Sauron himself stands or falls. Would not that have been a noble deed to set the credit of his Ring, if I had taken it by force or fear from my quest?
And now at last it comes. You will give me the Ring freely! In place of the Dark Lord you will set up a Queen. And I shall not be dark, but beautiful and terrible as the Morning and the Night! Fair as the Sea and Sun and the Snow upon the Mountain! Dreadful as the Storm and the Lightning! Stronger than the foundations of the earth. All shall love me and despair'

the phantom - you said that Sauron was not afraid of being overthrown, but of delay. These quote's say otherwise. Sauron could be overthrown if a great one of M-e should stand up wielding the Ring. Yet the fact that the Ring would corrupt the heart and mind of the one wielding it at that time, is another thing. That would not help Sauron.
Sauron would be overthrown and the one who overthrew him sets himself on his throne in Barad-dur, declaring himself the new Dark Lord. The people of the free realms of which he was an allie, otherwise he could not have wielded the Ring in the first place, will now turn against him and try to destroy him. Folly of course, for he is Ruling with the Ring at that time. So he puts the realms of M-e in a second Darkness. Sauron did not, the free people did not win, and the one on the Dark throne didn't win either for it was not his intention to become Dark Lord. It's the Ring that will win in the End if someone should stand up.

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

Oh, it might be. But point is that the other two Hobbits are missing, and therefore it is not sure where the Ring is. Saruman has good wits, the chance that the Ring was there, was of course greater (especially when you think of the rest of the company hunting after them and so on), but Saruman also knew pretty well that the Ring didn't have to be there.

The Hobbits could've died of course, but how and where? The orders of the Orcs were: 'kill all but not the Halflings. They are to be brought back alive and as captured, no spoiling and as quickly as possible'. The Orcs of the Tower of Cirith Ungol seem to have the same orders, so dying seems a bit hard, except by natural ways.

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.

But it can have merit, depending on the one who claims it. The ones who tried to claim the Ring for their own in LotR are too weak and therefore it had indeed no merit. But the ones who had the power, knew the consequences, and therefore were afraid of the Ring and did not make the claim.