What is the difference between "أبويه" (his two parents/father and mother) and "الوالدين" (the two parents/father and mother), and why was the word "الوالدين" not used in His (the Most High) saying: "And he raised his parents to the throne," while it was used in His saying: "And your Lord has decreed that you worship none but Him, and to parents show kindness"?

The words "al-walidayn" (the parents) and "al-abawayn" (the two fathers) have the same meaning, and there is no issue with varying the style in their usage. Some exegetes have mentioned that Yusuf's mother died when he was young, and that Ya'qub's wife was his maternal aunt, and thus not a complete mother. However, it is better not to overcomplicate the matter and simply attribute it to stylistic variation.