What should I do about my husband's anger over my reducing visits to his father, especially after his repeated abuse towards me?

You are not obligated to obey your husband regarding visiting his father or reducing visits, but your obedience to him is a part of good companionship. Explain to your husband what happened with his father, and apologize to him for the frequency of visits. If he accepts your apology, then there is no blame on you for reducing visits. If you wish to obey your husband seeking reward, then there is no harm in breaking your oath and making expiation for it; rather, it is better. This is based on the saying of the Prophet (peace be upon him): "By Allah, if Allah wills, I will not swear an oath and then see something better than it except that I will expiate for my oath and do that which is better, or do that which is better and expiate for my oath."