Is it permissible to seek forgiveness for deceased parents using the formula "أستغفر الله العلي العظيم وأتوب إليه، لي ولوالدي" (I seek forgiveness from Allah, the Most High, the Most Great, and I repent to Him, for myself and for my parents), and will repentance be accepted for them while they are in the abode of truth (the afterlife)?

May Allah reward you for your eagerness to honor your parents. There is no objection to your seeking forgiveness for them using the phrasing you mentioned, but it is better to delay "and I turn to Him in repentance." There is no specific time for seeking forgiveness, but supplication during times when prayers are answered is more likely to be accepted. There is no way to know their full contentment, and you must strive diligently in honoring them.