Keltham finds somebody else to cast his unwitting Wish. There's some pretty shitty seventh-circle casters in the world, who nonetheless don't want Rovagug consuming everything they've known—if you're willing to go completely to town on them, with mind-magic and Suggestions and mindscapes and false memories, until they'll cast a blinded Wish with the right utility function to be inverted.
It's not easy and it's more risky and the whole world is at stake and one person's feelings shouldn't matter that much; but even so, that Keltham finds that to use Carissa for that purpose would be doing too much violence to the last surviving bits of himself.