: 1. Who was Erestor?

One of the councillors of Elrond's household. Although in theory that should mean he was no slouch, he doesn't strike me as being particularly bright in the book. Of course, a lot of people don't think Celeborn seems all that wise, although I feel he does, so it's largely a matter of perception.

Somewhere, perhaps in MORGOTH'S RING or one of the later HOME volumes, Tolkien discusses who the "Elven Wise" were. They were essentially the lords of the Eldar. Erestor might or might not have been one of them. Glorfindel probably was on the White Council.

I note he does have Gandalf say, in "Many Meetings", that "here in Rivendell there live still some of hs chief foes: the Elven-wise, lords of the Eldar from beyond the furthest seas." Since most of Elrond's people appear to be Noldor, Erestor was quite possibly a Noldorin lord, perhaps born in Aman. That would make him a candidate for inclusion in the Elven-wise, and thus for inclusion on the White Council.

: 2. Were the blue wizards ever a part of the council, or were
: they gone east by the time the council was formed?

In one passage Tolkien says the Coucil included all the Istari but elsewhere he says it only included Saruman and Gandalf. You pays your money and you picks your horse.

: 3. Were any leaders of men ever part of the council? That's the
: part that confuses me most. I don't remember ever reading
: anything that suggests they were included. I would think that
: with the might of the elves fading and men growing stronger,
: that they (i.e. those of Numenorean descent) would be naturally
: included in trying to stop the growing shadow.

The Council was said to consist of Eldar and Istari. That would preclude any Men serving on it. I do note, however, that Tolkien referred (in one of his letters, I think) to the White Council as the chief or overall governing body of the Free Peoples in their opposition to Sauron. Not intended to convey a government, but rather the highest body for policy making. That would imply that Gondor (and not just the Dunedain of Arnor) would have been in contact with the Council. Faramir does tell Frodo that Men from Gondor have gone to Lorien, but the event has become rare by their day. It seems plausible the Stewards sent emmissaries to Lorien on occasion to consult with Galadriel and/or perhaps the White Council (which seems to have met there).

That's all largely guesswork, of course.

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Parma Endorion: Essays On Middle-earth