Why can't he read their minds? Varys and Bealish? This feels like extreme forced plot so he doesn't and won't find its them. Stupid, he got the Bolton from across the country (teleported) but he still had to know about the letters.
The MC needs to arrange a meeting between Viserys and the Martells. Also he needs to somehow show the people of Dorne how he planned the death of several of his own lords.