1) Is the Witch-King the same guy as the chief Nazgul that attacks Frodo on top of Weathertop?

2) If so - why didn't Frodo's sword (taken from the Barrow) damage this Nazgul baddie the way Merry's sword (also taken from the Barrow) did?

3) If the Witch-King is NOT the chief Nazgul that attacks Frodo on Weathertop - then who the heck WAS IT that got at Frodo in the FotR?

4) And, just so I'm clear on this (and it's obvious that I'm not), the Witch-King is not the same guy as the Mouth of Sauron, right?

I know - these are terribly basic questions - and I apologize for asking. But, I just needed to clear this up once and for all.

Thanks!