Imagine a scenario in which Martin wasn’t the reason for Jon’s last mark, and instead Jon got so isolated that the Lonely consumed him all by itself.

He was already an outcast; it would take just a little nudge for him to succumb to the Lonely altogether.

Jon was so painfully alone in season 4. Forsaken. Everyone gave up on him.

What I’m saying is that if Barnabas Bennett could send a letter to Jonah from the Lonely then Jon could surely receive one.