Is adultery restricted to married women, and is sleeping with an unmarried woman without a marriage contract not forbidden by Islamic law, and is "Mut'ah" marriage permissible, and its prohibition merely a jurisprudential interpretation with no basis in Islamic law?

The statement "There is no Sharia opinion with evidence that prohibits lying with her without a marriage contract" is incorrect, because the texts prohibiting zina (unlawful sexual intercourse) are general for anyone who is not lawful to a man (other than his wife or what his right hand possesses). There is no distinction between a married woman and others, even though zina with a married woman is more severely prohibited. Furthermore, Nikah al-Mut'ah (temporary marriage) is forbidden in Sharia.