It hasn't actually occurred to Keltham to worry about his possession not speaking! It feels fine to him too! If he wants her to exhibit any behavior he'll force it out of her!
He'll click the tickling collar around Carissa too (it's not her first time with it), and see about wringing some reactions out of her, screams and flinches, hysterical laughter, and also pleasure.
The only thing he'll say to her is that she's just there to be a thing that has real feelings, and not to try to force anything; he'll hurt her severely any time he sees her getting tangled up in her thoughts again. Just be in the chains, have no goals, and feel; anything resembling strategy is his own concern as her master. If they don't get a 100% total success on this occasion, Keltham will celebrate the progress they've made on their relationship and then write Abrogail for more advice.
So, no need to worry!