Whether the Nightfort or the Crossing were truly cursed Jon could only say if he visited them personally. But if he ever did so with the latter, he would instead pull the castle down stone by stone and salt the surrounding land, then have someone hold it in his name. The North had plenty of empty lands, it did not need any more. If he truly wanted more land, he could take back the Gift. It was only slightly less than half of the Riverlands and was almost as fertile, and way closer and far more defensible.