[Suzerain of Sheol]

I'm fairly certain the White Council was aware of the nature of the Ring-Wraiths. In the books, Glorfindel was also on the Council, and he personally fought against the Witch-King when they drove him out of Angmar (He's actually the one that spoke the "Not by the hand of man shall he fall" prophecy), even if the others were ignorant (which I find unlikely), he would have known, I believe. But Saruman had made it his great purpose to study everything he could relating to Ring-lore, so I'm pretty sure he, too, would know how the Nine were created.

About the Necromancer, they suspected it was Sauron, or feared it was, but they didn't know for sure until they drove him out.

We'll have to wait for at least the next movie to see how this sub-plot plays out, but it looks to me at this point that they're inventing most of it (to be fair, they don't have legal access to the other books, so they kind of have to). It's just a shame, to me, because there's a really rich backstory about the wars with Angmar (which IS in the appendix of Lord of the Rings, which they're allowed to use), but instead they seem to be making up their own version of past events. I don't expect them to put the book word-for-word on the screen, and if they could invent stuff that was BETTER than the book, I'd be all for it, but... almost every time they've done something like this, to me, it's always been a change for the worse and left me wondering why they didn't just use Tolkien's original idea.