What is the expiation for making up 800 days of Ramadan fasting for someone who missed them over their lifetime, and is it a condition to feed 800 needy persons, or is sincere repentance sufficient, along with knowing the way to achieve it, and should feeding the needy be done at any time or only during Ramadan?

You must repent and make up for the days of Ramadan you missed, along with feeding the poor for the delay. You must also offer expiation for intercourse if the fast was broken due to it (this applies specifically to the man). Let all of this be done to the best of your ability. If death overtakes you before you complete it, we hope that you will be excused by Allah, as long as you have repented and your repentance is sincere.