I understand why Martin wanted the Greyjoys to attack, but he never establishes their motivation. In fact, it makes no sense at all, and the attack predictably fails despite the Greyjoys having completely caught Robb Stark by surprise and left him helpless to do anything other than roving bands of northerners. And even if somehow the Greyjoys could hold onto some part of the North, would Tywin ever have let them remain independent? Of course not. It is actually a plot hole that he doesn't crush them once he has Robb Stark murdered. It was certainly never explained why Tywin allows them to continue to function independently at all. Even Robert Baratheon crushed them when they rose against him. It is like Martin knew he made a mess of that plotline, but he wanted to do other stuff with them so he lets them stay out there despite the fact that it undermines the rest of the story.
I don't know if you have read the books, but Martin makes a total mess of that plot line by the end of the fifth book.
It was guaranteed to fail for the Greyjoys, which undermines everything else that they do, yet somehow they emerge under Euron stronger than they were before losing most of their army in the North? I think it qualifies as a huge plot hole, and undermines everything that comes afterward in the books and the show.
I think the plot tangle that Martin had created for both Sansa and the Greyjoys is the reason that he will never finish the books. There is no way out from where he has the story.