[Scurveydog77]

You make a good point, but why would Martin put it in the book? He offers it in two different scenes. It could be that Rob changed his mind and made someone else his heir, they never mention Jon's name when they put their seals to the document.

Also, keep in mind that the Lord Commander of the Night Watch already has lands and castles. Castle Black, The Gift, The new Gift, Queens Castle, etc.

And not all those who signed the document are dead or captured. Rob sent some of them to search out Howland Reed. They never went to the Red Wedding. One of them was the Lady Mormont. She'll know the name of Rob's heir.