Higher order: Gandalf or Balrog(s)

[Tuor Turambar,Cursed by the Valar]

not powerful enough to overthrow Sauron themselves

Are you saying that the Valar could not have ovethrown sauron? If they were powerful enough to overthrow Morgoth, I think they could overthrow sauron. And what why send the istari, if not to help overthrow morgoth?
If you are going to send some wizards to overthrow sauron, and they have not restrictions (you think I was cart-before-the-horse), then why not attack with your whole army? If you are trying to avoid a repeat of the war of wrath, then you have to put restictions on the maiar you send.
And if there were no restrictions, what was wrong with Saruman becoming a lord, if you will? I am assuming you also agree that what Saruman did was AGAINST what the valar wanted.