Saruman for or against Sauron?

[Raynor]

Please explain how a physical confrontation with Sauron could possibly be avoided by this putative ringlord.

I believe that this could be achieved by the way Elrond/Galadriel would have tried: large armies with absolutely subservient generals. I don't think that killing Sauron by 'mere' warriors is out of the question, seeing that it had already happened once at the end of the second age, in direct combat.

How could said ringlord be sure they had mastered the Ring when they fought Sauron?

I believe that said ringlord would have avoided direct confrontation; as it has been quoted already in the thread, and argued by you, such a direct encounter would greatly favor Sauron.