Is a wife considered disobedient (nashiz) for leaving the marital home without her husband's permission, after being subjected to physical and verbal abuse, and his threatening her that he would masturbate while watching pornography, especially given that she is eight months pregnant and has a young daughter?

If you left your husband's home due to his severe abuse, you are not considered "nashiz" (disobedient/rebellious). This is permissible for you according to the consensus of jurists if your departure was due to an affliction or severe beating. However, if your departure was without justification, then you are "nashiz" and must repent and return. Your maintenance is obligatory upon your husband if you are not "nashiz." The most authoritative opinion is that it remains obligatory until childbirth even if you are "nashiz." Retaliating against your husband's mistreatment in kind is often harmful. Obedience to him regarding the insertion of an IUD is not obligatory if there is harm. You must advise him to stop watching pornography and masturbating.