Which of the two phrases is better to use in supplication for parents, for the worshipper, and for Muslims: "الوالدين" (the two parents - dual) or "الوالدين" (parents - plural), and what is the evidence for the preference?

The word "al-walidayn" (الوالدين), with an open "dal" (دَ), indicates the dual form, not the plural. This is how it appears in the Quran on the tongues of Noah and Abraham, peace be upon them. To supplicate for one's parents, one should pray with what is mentioned in the Quran. If one wishes to include others, they may say: "Our Lord, forgive us and our parents and all Muslims," or "O Allah, forgive us and our fathers and our mothers and all Muslims," or "Our Lord, forgive us and our parents" (والدينا) with a broken "dal" (دِ). The door of supplication is wide, and it is better to include Muslims in it, especially for an imam, as an imam has been forbidden from singling himself out in supplication without the congregants.