Saruman for or against Sauron?

[Kuruharan]

And I believe that what Elrond said, that by using the arts of the enemy one of great power could ''overthrow'' (& not just challenge) Sauron & take his seat on his throne to become yet another Dark Lord is not just a mere thought or prediction, it is the truth. Elrond afterall is greatest of all the loremasters, & is telling it as it really is

We may just have to disagree on that, as that is an interpretation of the text. I believe my interpretation is more in line with what Tolkien wrote elsewhere and Tolkien’s writings are the standard I go by.

Sauron knows (& he should do - he is the lord of the Ring afterall!),

Ahhh, yes…let’s talk about what Sauron “knows” about the Ring…

And this is the dreadful chance, Frodo. He believed that the One had perished; that the Elves had destroyed it, as should have been done.
-The Shadow of the Past

…evidently not much. If he didn’t know that he would have been utterly destroyed by the destruction of the Ring, I don’t think we can place ultimate reliance upon his supposed knowledge of who could or could not use the Ring to challenge him.