This is spoilerville, so be warned.
1. Robb IS a fucking moron, especially the way the 'romance' is depicted in the show. Essentially she is the only woman around so he decides he has to marry her and spit in the face of his only real ally in the south.
2. Jon Snow is probably Lyanna's son by Rhaegar Targaryan. I wouldn't expect you to have any inclination towards this theory though as the show has avoided much of the key back-story from the books.
5. It is a combination. I didn't much care for the character in the books myself, but the actress (and the dialogue they give her) is one of the notably weak links in the show.