Is it permissible for a woman to refuse to return to her husband after he has divorced her twice, especially since she does not wish to return to him for fear of him issuing the final divorce?

Allah has commanded good treatment of wives. You have mentioned things about your husband that contradict the sharia’s directives, and you have done well by being patient with him. If you are still in your iddah (waiting period after divorce), he has the right to take you back. If you strongly believe he will reform, then accept. But if you strongly believe things will remain as they are, then seek a khul (divorce at wife’s instance). If the iddah has expired, he cannot take you back except with your consent, and you may refuse to return to him; you will not be unjust to him.