Is the ex-husband of the questioner's former wife and the mother of his children considered a mahram to his daughter from his current wife, and is it permissible for his daughter to greet him and sit with him without a hijab?

Your daughter is a foreign woman from the wife of your former children's father, so there is no relation between them. Therefore, she is not permitted to sit with him without a head covering if she has reached the age of puberty (when wearing the hijab becomes obligatory). However, it is permissible for her to greet him and for him to greet her, provided there is no fear of temptation arising.