Were the Istari Unable or Unwilling...

[Sardy]

...to eschew their mortal bodies? As the five wizards were bound in human, corporeal form, was this a choice of their own, or a mandate passed down from above (of which they were unable to reverse)?

Gandalf, of course, would 'stick to the rules of the game' and not "revert" to a more "godly" form, even in the most dire of circumstances... but Saruman? I wonder what Saruman's memory of his Maiar status/powers/abilities might have been?

A subset of this question would be, what (if anything) did Gandalf retain from his immortal days in Valinor? Did he actually remember a time before he was "Gandalf"? Did he have a full, waking knoweldge of his (former?) status, nature and abilities?

Edit: In the title of the thread, I changed "Maiar" to "Istari", as that is more apropos of the topic...