[narfforc]

I think what The 1,000th Reader alludes to is written in Letters 156 and 181, where Tolkien states that Gandalf returned with enhanced power. Tolkien also wrote this revealing passage:

So Gandalf sacrificed himself, was accepted, and enhanced, and returned. 'Yes that was my name. I was Gandalf.' Of course he remained similar in personality and idiosyncracy, but both his wisdom and power are much greater.
The authority was given to subdue Saruman.

In The Lord of the Rings Companion Note 758 has this interesting comment:

Gandalf's rejoiner.....carries a great many implications in a few words. He is reminding Denethor that he [Gandalf] is a representative of a higher authority than Denethor is (i.e., The Valar and/or Iluvatar). He is reproving Denethor: 'You are answerable to a higher authority.' He is implying that Gondor is part of his own stewardship too, and finally he is pointing out that Gondor is only part of a larger battle.
[Romenna Meeting Report, p.2].

In Letter 286 Tolkien writes: There are no 'Gods', properly so-called, in the mythological background in my stories. Their place is taken by persons referred to as the Valar (or Powers): angelic created beings appointed to the govenment of the world.

In Letter 325 Tolkien writes: The angelic immortals (incarnate only at their will), the Valar or regents under God.........

The meaning of the word regent I take to be thus:
Regent: Ruler of a kingdom during the absence of its Monarch.

I therefore believe that Gandalf/Olorin is a steward of Eru.