[Earendilyon]

Originally Posted by The Saucepan Man

Which would mean, in a way, that Sauron was the ultimate winner. And it seems to me, although Tolkien does not say as much in that letter, that this would open up the possibility of Sauron regaining physical presence and returning to claim the Ring from the Ring-mastered Gandalf.

I beg to disagree on this point. As I see it, Sauron put a lot of his power into the Ring, but not himself! After the Ring was indeed destroyed, Sauron did still exist, but completely powerless, because he had put most of his power into the Ring, and much of the rest into restoring himself and Mordor a couple of times.
The effect of Gandalf mastering the Ring and overcoming Sauron with it, would be the same to Sauron as when it were destroyed: its power would forever be lost to him:

If Gandalf proved the victor, the result would have been for Sauron the same as the destruction of the Ring; for him it would have been destroyed, taken from him for ever. - Letter # 246

The Ring would continue to exist, though. And the ultimate effect for Middle-earth would be disastrous, because:

Gandalf as Ring-Lord would have been far worse than Sauron. He would have remained 'righteous', but self-righteous. He would have continued to rule and order things for 'good', and the benefit of his subjects according to his wisdom (which was and would have remained great).
[The draft ends here. In the margin Tolkien wrote: 'Thus while Sauron multiplied [illegible word] evil, he left "good" clearly distinguishable from it. Gandalf would have made good detestable and seem evil.']- Letter # 246