[Kuruharan]

I would quibble over giving too much weight to the quote about "only Gandalf".

I don't see how it would be possible to do so.

I see no reason why Gandalf should have been able to use the Ring, and not Saruman.

-and-

my reading of the books always left me with the idea that Gandalf, Elrond, or Galadriel had pretty much equal chances of becoming successful Dark Lords (or Lady) had they succumbed to the Ring's evil. Consequently, I have no difficulties seeing Saruman as being able, had he acquired the Ring.

These are all merely your opinions, which are in stark and direct contradiction to the expressed opinion of the author of the works. You can certainly argue with him about anything you like, but once you start disagreeing with things he says would have happened in the story you are no longer talking about his works, you are talking about yours.

Because I do now see a bit of ambiguity with Letter 246. 'only Gandalf might be expected...' So even though if Gandalf is the only one 'expected' to challenge Sauron one-on-one, it doesn't necessarily mean he was the only one capable of doing so.

Again, I'm just completely failing to see how that statement admits of any other possible interpretation than only Gandalf could master the One Ring. The "might be expected" clause only reinforces the idea that even in that case it is not a sure thing that Gandalf could win, but Gandalf is the only one who had even a chance of winning.