Is saying, "You are to me as my mother were you to enable me to yourself in this way again," considered ẓihār or a slip of the tongue, and is it permissible to switch to the expiation of feeding due to the two-month fasting coinciding with Sha‘bān and Ramaḍān, and what is the required amount for feeding?

This act is reprehensible, and you must repent sincerely. Your statement to your wife, "You are to me as the back of my mother," is objectionable and false. If she allowed you to approach her after this statement and you did not compel her, then ẓihār has occurred. It is not permissible for you to approach her until you perform the expiation for ẓihār in the prescribed order: freeing a slave, and if you cannot find one, then fasting for two consecutive months, and if you are unable to do so, then feeding sixty poor people. The continuity of the fast is not broken by fasting Ramadan or by breaking the fast on Eid day or similar occasions. The amount of food for each poor person is one mudd of food (750 grams), meaning 45 kilograms for sixty poor people. It is preferable to give it as food, unless there is a benefit in giving its monetary value.