PRNK (Or anyone else for that matter) only has to get a workable SHORT RANGE nuclear tipped missile and sail it up close to Hawaii or the west coast on an old fishing boat. Not too far fetched a possibility.
Why bother with the missile part? Just put it in a third-party-flagged freighter and sail it into the US harbor of your choice.
Yes and no. To do that, they’d need a different guidance system. You can’t just put any ballistic missile on a ship and have it perform the same.