[The Saucepan Man]

After all, bound in the body he was, he certainly could not oppose Sauron in a test of whatever strengths the Maiar had.

The Istari were forbidden to use their Maiar powers while engaged in their mission on ME. But how binding was that prohibition? Could they ignore it? And what would be the consequences of doing so? If they were able to do so, and remain in ME, then there's no reason why Saruman, having already turned away from his mission, would not do so. Then he would be a match for Sauron, surely? And even without their Maiar powers, the Istari were still pretty powerful. I am not so sure that, even then, Sauron would consider Saruman to be no threat, particularly if there was any chance of him getting the Ring.

Did "human wizards" actually exist in Tolkien's universe?

Well, there were no human "wizards", since that term was used to denote the Istari. But there were certainly human sorcerors. The Witch King was one such, before becoming a wraith.