Is there a sin incurred by the fiancé if his veiled fiancée refuses him, or upon her if she is unable to veil herself, and is it true that Allah did not show the Prophet Muhammad, peace be upon him, the torment of unveiled women?

We have not come across any indication that the Prophet, peace and blessings be upon him, saw the specific punishment of women who do not wear the hijab. As for his saying, "Two types of people from the inhabitants of Hell whom I have not seen," its meaning is that he had not seen them in his time in this world, and it is not evidence that he did not see them during the Isra and Mi'raj or in his sleep.