A few questions about Gandalf not fighting Sauron

[The Might]

Hi Glorthelion,

the problem ist that a direct attack on Barad-dur and on Sauron, no matter who lead it would probably be destined to fail.
Tolkien explores this question in letter #246:

Of the others only Gandalf might be expected to master him – being an emissary of the Powers and a creature of the same order, an immortal spirit taking a visible physical form.

Confrontation of Sauron alone, unaided, self to self was not contemplated. One can imagine the scene in which Gandalf, say, was placed in such a position. It would be a delicate balance. On one side the true allegiance of the Ring to Sauron; on the other superior strength because Sauron was not actually in possession, and perhaps also because he was weakened by long corruption and expenditure of will in dominating inferiors. 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. But the Ring and all its works would have endured. It would have been the master in the end.
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).

So a direct duel without possession of the Ring would be impossible to win, and a duel wielding the Ring would still not yield a positive result, as Gandalf "would have been far worse than Sauron".