Is a single expiation required for repeated breaking of a single oath, and what is the ruling on repeatedly fasting as an expiation without knowing the obligation of feeding the poor?

This oath does not necessitate repetition. So, whenever you break it and fail to recite Surat Al-Qasas on a night, the oath is dissolved, and a single expiation becomes binding upon you. The expiation does not repeat thereafter due to abandonment. You have the liberty to discontinue reciting the surah after the expiation. As long as you have expiated for the oath by feeding ten needy people, you are absolved of responsibility, and nothing else is required of you thereafter.