Rough Skin & Iron Barbs replace Rocky Helmet, since they cannot be k-offed.
No, Knock Off is banned. Items that provide Abilities are not immune to being removed. Rocky Helmet also hits harder -there's not actually any reason to run the Rough Skin or Iron Barbs items, as they're strictly inferior.
Rapid Spinners have Scrappy to bypass ghosts.
-ate works too, and is honestly probably better, providing a 30% power boost and the potential to hit super effectively. Not that people use Rapid Spin for its damage... but having it do better damage may well make the difference. Aerilated Rapid Spin could be a nasty surprise for Leech Seeders, for instance.