You need to play Crisis Core to properly understand why he knows of Nimbelhelm.  At least, I think that's correct.

No... I think? Christ, the storyline for VII has completely vanished from my memory for the most part.

 Cloud's not a clone of Sephiroth's. It's true that Sephiroth says this on multiple occasions, but it's later revealed that Cloud was just experimented on which put him on a level of a SOLDIER First Class (or above), but more or less left him lethargic at first and mentally unbalanced after. Sephiroth's been known to lie, claiming that "there may have once been a real Cloud", implying that FFVII's Cloud is just a fake.

It may be important to note that the Japanese have a different definition of "cloning", rather, that translation is fuzzy on the subject. Whenever altered DNA or genetical experiments rise to the surface, it's usually considered "cloning" - take your King of Fighters, for example*.    

Nope, Cloud is not a clone.