I don't really understand who the bone carver shows to others, when he was still in the prison. Rhys saw Jurian, Feyre saw her future son with Rhys, and Cassian we don't know. So... who does he show? Maybe I missed it in ACOMAF.
Well I don´t know for certain but he may be showing people the characters had, have or will have week spots for. Or just a person who doesn´t look as terrifying as his true form. Idk but I am certain that we will find out more about that in the next book (a court of silver flames).