[Child of the 7th Age]

If Maiar become bound to their hröar through indulgence in physical pleasures (eating, drinking, begetting etc), does that mean that Melian and Gandalf (and any other of the Istari that may have returned) remained bound to their physical form on their return to Aman? Or, once back in the Undying Lands, could they relinquish their physical form and once again assume "spiritual" form?

SpM - Speaking directly to the question you've raised.... Yes, I've read that quote from osanwe before, but have always felt that "being trapped" was a danger upon misuse of the form, rather than something inherent to the form itself.

HI's own osanwe quote suggests that the problem stems from abuse:

Melkor alone of the Great became at last bound to a bodily form; but that was because of the use that he made of this in his purpose to become Lord of the Incarnate, and of the great evils that he did in the visible body.

Other writings suggests that the ability to take on a carnate form was a "natural" ability of a Maia and should not prevent him or her from ultimately transitioning back to the original form. Presumably, the only danger would come in the "abuse" of that form. Remember that in the early years many of the Valar and Maier frequently came among the Elves in Elvish forms and went back to Aman, although the Elves did not know that.

UT says this:

The Maiar were 'spirits', but capable of self-incarnation, and could take 'humane' (especially elvish) forms.

The converse, it would seem, should also hold true. If there had been any inherent danger of not being able to shift back, Tolkien would have mentioned that in the section on the Istari in UT. He does discuss other dangers the Istari would face. They are told that the change to carnate form "would imperil them, dimming their wisdom and knowledge, and confusing them with fears, cares, and weariness coming from the flesh."(p. 393). But he doesn't suggest they'll never be able to "come home".

If the Istari had been told they would be trapped forever in a carnate body merely because they had eaten a meal or enjoyed a pint of ale, I doubt they would have agreed to come. And surely Manwe would not withold such an important piece of information from them, if it was true.

I do wonder if the fact that Gandalf could so easily communicate by osanwe suggests that his mind was still open and confirms that he had not fallen prey to the abuse of the body the essay suggests is possible. the essay speaks of the communications of the Valar having the "highest authority" and "greatest urgency". Those are exactly the kind of phrases that comes to mind when I think of Frodo's long-distance warning from Gandalf. So perhaps this is proof of the fact that Gandalf's body is just a covering and he is still a true Valar inside.

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Regarding changing of forms. I've always been under the impression that Radagast who is actually termed a "shapeshifter" at one point did still retain that ability in Middle-earth.