Am I entitled to seek divorce from my husband, whom I was forced to marry and who, as I discovered, has illicit relationships and wants to enter into a "misyar" marriage, especially since my family rejects my divorce and labels me as disobedient?

What we advise you to do is to return to your husband, and strive to reconcile with him and reach an understanding; to achieve the family's welfare, please your parents, and avoid divorce. If that is not possible, or if you dislike him to the extent that you cannot fulfill his rights, then you may seek khul‘ from him. This is not considered disobedience to your parents, and your mother’s prayer against you in this situation will not harm you, Allah willing, as long as you are dutiful to her. A parent’s prayer against their child is effective only if the child has wronged them.