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 walk 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 Maiar form, such that the option was never there to begin with? Or would he, even in Maiar 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 Maiar - 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 Ainur 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?