I seriously doubt Martin was talking about Stannis. Stannis already has seen his fate...
I know the cost! Last night, gazing into that hearth, I saw things in the flames as well. I saw a king, a crown of fire on his brows, burning
burning, Davos. His own crown consumed his flesh and turned him into ash. Do you think I need Melisandre to tell me what that means? Or you?
He also sees what he thinks will be the last battle:
With mine own eyes. After the battle, when I was lost to despair, the Lady Melisandre bid me gaze into the hearthfire. The chimney was drawing strongly, and bits of ash were rising from the fire. I stared at them, feeling half a fool, but she bid me look deeper, and . . . the ashes were white, rising in the updraft, yet all at once it seemed as if they were falling. Snow, I thought. Then the sparks in the air seemed to circle, to become a ring of torches, and I was looking through the fire down on some high hill in a forest. The cinders had become men in black behind the torches, and there were shapes moving through the snow. For all the heat of the fire, I felt a cold so terrible I shivered, and when
I did the sight was gone, the fire but a fire once again. But what I saw was real, Id stake my kingdom on it.
I'm going to assume Martin knows exactly what's going to happen to Stannis. Not to mention that Dany sees a vision of a blue eyed king who casts no shadow.
I also don't believe Martin's comments are about Barristan either, who I think is being set up by the Shavepate
I don't think he needs to marry Arianne for the reason you state. However when Arrianne learns what happened to Quentyn I expect her to put Dorne's full weight behind him, and attempt to seduce him. Whether it works who knows.