The White Council assaults Dol Guldur?

[Kuruharan]

Considering the shock of discovering a Balrog in Moria, I don't think that was ever considered.

Some other sorceror of human (or maybe even Elvish, but I find that almost impossible) origin might have been a possibility.

I think they must have assumed Nazgul from the beginning and only thought "Sauron" later on down the road.

[alatar]

Originally Posted by Kuruharan

Considering the shock of discovering a Balrog in Moria, I don't think that was ever considered.

Was everyone surprised at the nature of Durin's Bane? What did Galadriel know?

Some other sorceror of human (or maybe even Elvish, but I find that almost impossible) origin might have been a possibility.

I think they must have assumed Nazgul from the beginning and only thought "Sauron" later on down the road.

But what of Elrond? He knew that Sauron more likely was not destroyed. He was able to glimpse the future. Could he have not considered, even if unlikely, that the Necromancer was possibly his old friend from Mordor?

And why did Tolkien use the word, 'necromancer?' I understand (maybe) why he didn't use wizard or witch, but why the title that speaks of communicating with the dead, that more than Sorcerer?

[Legate of Amon Lanc]

Originally Posted by alatar

Was everyone surprised at the nature of Durin's Bane? What did Galadriel know?

Everyone was surprised, definitely, while Galadriel of course remains a little mysterious. While Celeborn was immediately shouting "Oh no! We knew there was something, but I never imagined...! Had we known that...!", Galadriel, if I remember correctly, was silent - and I can imagine her thinking "Of course, I know... but I did not tell them, after all, as long as it was not dangerous for us, why tell them?"

But what of Elrond? He knew that Sauron more likely was not destroyed. He was able to glimpse the future. Could he have not considered, even if unlikely, that the Necromancer was possibly his old friend from Mordor?

At some point before Gandalf came to Dol Guldur for the second time and finally confirmed the truth, he and Elrond at least presumed that. Maybe even others did, but the text tells us definitely that the two of them did (depends on how you want to interpretate "us" here, but you see they are here in contrary to "many"):

Originally Posted by Silmarillion; Of the Rings of Power and the Third Age

Now the Shadow grew ever greater, and the hearts of Elrond and Mithrandir darkened. Therefore on a time Mithrandir at great peril went again to Dol Guldur and the pits of the Sorcerer, and he discovered the truth of his fears, and escaped. And returning to Elrond he said:
"True, alas, is our guess. This is not one of the Úlairi, as many have long supposed. It is Sauron himself who has taken shape again and now grows apace; and he is gathering again all the Rings to his hand; and he seeks ever for news of the One, and of the Heirs of Isildur, if they live still on earth."
And Elrond answered: "In the hour that Isildur took the Ring and would not surrender it, this doom was wrought, that Sauron should return."

So yes, Elrond probably knew and thought about it, as well as Gandalf, but still it took them long to take some action. Well, the text implies that going to Dol Guldur was not much of a funny walk, so maybe they were reluctant to go even to spy there, if the majority of the Council did not want to give their hands to it (considering it probably just "some little Nazgul and it's not worth it risking your life by going to confirm it").

And why did Tolkien use the word, 'necromancer?' I understand (maybe) why he didn't use wizard or witch, but why the title that speaks of communicating with the dead, that more than Sorcerer?

I believe in some HoME there was something Tolkien wrote about it. The point was that Sauron was able to manipulate with the spirits of the dead, trapping the spirits of dead dark Elves unwilling to go to Valinor and similar things - I don't know personally, but it has been discussed here not that long ago, if you look around for similar topics, someone mentioned that (I believe Elmo it was, about a year ago, more or less).

[Gordis]

Originally Posted by alatar

And why did Tolkien use the word, 'necromancer?' I understand (maybe) why he didn't use wizard or witch, but why the title that speaks of communicating with the dead, that more than Sorcerer?

The term "Necromancer" for the Sorcerer of Dol Guldur was devised in 1930-ies, when Tolkien was writing "the Hobbit" - long before Tolkien invented Sauron or the One Ring.
At the time, and almost all through Tolkien's work on the LOTR, the wizards were not what they have become later - not Maiar, but mortal Men "of a noble profession", wielding sorcery and having a very long life.

Some remnants of these earlier ideas are still visible in LOTR. In the “Shadow of the Past”, Gandalf expresses a "professional” interest in the Ring: " I was professionally interested in your ring, you may say; and I still am.”

At first the Wizards were supposed to be rather numerous and common - almost like special "folk".
See this part of the LOTR prologue:

And certainly it was from Bree that the art of smoking the genuine weed spread in the recent centuries among Dwarves and such other folk, Rangers, Wizards, or wanderers, as still passed to and fro through that ancient road-meeting.

Compare it with the quote from the Hobbit:

Indeed Bilbo found he had lost more than spoons – he had lost his reputation. It is true that for ever after he remained an elf-friend, and had the honor of dwarves, wizards, and all such folk as ever passed that way; but he was no longer quite respectable.

The wizards were supposed to come from Numenor. See this quote from the LOTR ( also present in early drafts):

'Who is Saruman?' asked Pippin. 'Do you know anything about his history?' 'Saruman is a Wizard,' answered Treebeard. 'More than that I cannot say. I do not know the history of Wizards. They appeared first after the Great Ships came over the Sea; but if they came with the Ships I never can tell. Saruman was reckoned great among them

Now in “the Hobbit”, how to single out a “bad” one among numerous human sorcerers? - Referring to him as “Necromancer” implied that he used Black Magic, meddled with the spirits of the Dead- a bad thing that the other wizards didn’t do. I don’t think that in the 1930-ies it went any deeper than that.