Control over The One Ring

[Aiwendil]

I do (and did) realize that Tolkien explanation there seemed lacking, but he must've been keeping it relatively short for the letter's sake. The fact that he did say 'only' still indicates to me that Gandalf was the only period.

True. One further argument I could make would be that perhaps the possibility of one using the Ring to defeat Sauron would depend not only on one's overall power, but more particularly on one's foresight, wisdom, strength of will, etc. - qualities in which Gandalf surpassed Saruman. So I still think that the statement can't be used to prove that Gandalf is, overall, superior to Saruman. But I acknowledge that it makes a good argument.

If he is referring only to the Wise, I do suppose that would leave it open to the other Istari, though CT and Saruman observe Radagast as a being of lesser stature.

Yes. I am (as you guessed) a great fan of Radagast (and the Ithryn Luin to a lesser degree) but certainly Saruman and Gandalf were above the others.