Would a son be sinful if he treated his separated parents equally in his dutifulness to them, despite his mother's insistence that he favor her, basing his stance on the fact that the repetition in "your mother, then your mother" is for emphasis and not favoritism, and that the verses of the Qur'an equate both parents in dutifulness?

It is necessary to refer to the understanding of scholars to know the meanings of the texts of the Qur’an and Sunnah (definition), because they are the people of authority who deduce rulings. To understand the text of the Qur’an regarding prioritizing the mother over the father in cases of conflict, the Qur’an and Sunnah have indicated this. Moreover, the repetition in the commandment concerning the mother implies emphasis. The original principle remains reconciling between both parents, but if reconciliation is difficult, then the mother's right takes precedence over the father's right due to the additional care given to her by the Shari'ah. A Muslim must honor both parents, be just between them, and be patient with the mother.