I feel fairly certain that he won't marry Dany and i actually don't think Dany would see this as necessary to cement an alliance, although Aegon's camp may want this to add legitimacy, but to some extent so would being accepted in marriage to any of the great houses. Whether she is barren or not, Dany thinks she is. Unlikely she will want to risk the future of her family by marrying the only male heir if she is unable to have children. If she returns to Westero's at all of course.
I'm still thinking Ramsey wrote the letter but i do find some of the conspiracy element compelling and I'm definitely not 100% convinced of the true author. This whole thread has got me thinking about the timelines of Davos meeting Manderly and leaving (we assume for Skagos) and Manderly moving to Winterfell. Could Davos have found Rickon and Manderly knows this before his "showdown" with Ransey leading for him to ride out against Stannis. Remember Manderly swore to support Stannis if he had Rickon. I then think about what Stannis said "It may be that we shall lose this battle," the king said grimly. "In Braavos you may hear that I am dead. It may even be true. You shall find my sellswords nonetheless." I've always taken this to think that Stannis was planning some subterfuge for the coming confrontation. He wants to make sure he get's his sellswords even if he's dead. Manderly is cunning. I think he goaded Ramsey and Roose into a trap of Stannis's making. The winner is unknown. Anyhow i digress...