Is a wife considered to be unjust if she invokes [evil] upon her ex-husband who severely mistreated her and her children, especially given that he is currently threatening to take the children after she sought a divorce from him?

We advise you to turn to Allah Almighty in all your affairs, and to be diligent in remembrance (dhikr) and supplications for distress. You have done well to inquire about the man's religiosity and character. Indeed, one who preserves prayer is hoped to have Allah's care in his character, whereas one who neglects it may commit bad actions such as striking and miserliness. The husband's miserliness and failure to spend are forbidden, and it is permissible for you to take from his money what suffices you and your child in a reasonable manner. If his mother lied in her praise of him, this is deception and fraud. If what you mentioned about registering the land in your husband's name without your permission and his taking your money deceptively is true, then you have the right to take the matter to court. Your husband has no right to threaten you with returning to him or taking the children, as custody is your right as long as you do not remarry. You are oppressed, and the supplication of the oppressed is permissible, though forgiveness is better.