Why didn't Gandalf carry the Ring in Maia form?

Question

I understand that the Istari - the only other Maiar besides the Balrog to come into close proximity to the Ring - are constrained within a mortal body and subject to its mortal weaknesses, which renders them vulnerable to the Ring's domination and corruption. However, what is to stop Gandalf from returning to his true form, picking up the Ring, and just walking all the way to Mount Doom? Sauron is too weak at the time; he would have been able to do nothing but watch helplessly and pray his mortal armies can defeat a Ring-bearing Gandalf.

Is Gandalf's decision to abide by the Valar's will to the end the only thing holding him back from this particular option? Are there enchantments in place to disable his ability to revert to Maia form, such that the option was never there to begin with? Or would he, even in Maia form and free of mortal weaknesses, still be vulnerable to domination and corruption by the Ring?

The One Ring is forged with the essence of Sauron to dominate and rule the minds of lesser beings. However, Sauron is only a Maia - albeit the highest-ranking in Morgoth's following. Fellow Maiar would be his equal (at least in rank), while the Valar are certainly superior in power to him. If an Ainu becomes a Ringbearer, will he/she eventually fall to Sauron's will too, or at least succumb to the corruption of the Ring's dark power?

Answer 1

From Tolkien's letters, Letter 156 (emphasis mine):

When he speaks he commands attention; the old Gandalf could not have dealt so with Theoden, nor with Saruman. He is still under the obligation of concealing his power and teaching rather than forcing or dominating wills, but where the physical powers of the Enemy are too great for the good will of the opposers to be effective he can act in emergency as an 'angel' – no more violently than the release of St Peter from prison.

(Letter 156, incidentally, contains a great deal more information about Gandalf, his Maia nature, and his transformation from Grey to White. You can find it quoted at much greater length here.)

And from the essay on Istari in Unfinished Tales (again, emphasis mine):

their emissaries were forbidden to reveal themselves in forms of majesty, or to seek to rule the wills of Men and Elves by open display of power, but coming in shapes weak and humble were bidden to advise and persuade Men and Elves to good, and to seek to unite in love and understanding all those whom Sauron, should he come again, would endeavour to dominate and corrupt.

So yes, Gandalf is still obliged to take a physical form - to be "incarnate" as Tolkien put it - rather then revert to his true angelic Maia form. And even after his reincarnation as Gandalf the White, Gandalf is still weaker than Sauron. He says so explicitly in The Two Towers, chapter The White Rider:

"I am Gandalf, Gandalf the White, but Black is mightier still."

And the power of Gandalf would have drawn the attention of Sauron much faster than two puny hobbits. If Gandalf had claimed the Ring and walked into Mordor - perhaps even if he had taken Maia form in order to do so - then matters would soon have come to a confrontation with Sauron: a confrontation which Gandalf, despite his power, might not have won.

Note also that by the time he returned as Gandalf the White, the Fellowship was broken and the Ring was already well on its way towards Mordor. The only time Gandalf could have made the decision to take Maia form and bear the Ring himself was while he was still in his weaker form of Gandalf the Grey.

Answer 2

While I don't disagree with anything @randalthor wrote, there is a much more important issue preventing Gandalf the While from just walking to Mount Doom and destroying it, and it's the same one that prevented Gandalf the Grey from doing so. One of the Ring's chief powers is to corrupt those who wear it, turning them to evil. Being more powerful doesn't make you harder to corrupt, in fact there are many indications that being more powerful makes you easier to corrupt. It's one of the central points of the book. Gandalf the White would not have been more immune to its effects than Gandalf the Grey. It's at least part of the reason why the Hobbits survive so long as Ringbearers, while others get affected more quickly. Both Boromir and Denethor are corrupted by it without even being ringbearers.

We know this at least because of Gandalf's attitude to the Ring. When offered the Ring he absolutely refuses it, preferring that it stays with the 'weaker' Frodo.

Answer 3

It says in Unfinished tales ‘ the Ainur were forbidden by Illuvatar to walk among men in forms of majesty’ so that would include Gandalf, orders from Eru would be even more important than stopping Sauron.