Why was the tenth of Tishri chosen as a day of forgiveness?

The Gemara (Yoma 20) teaches that "the Satan" (=364 in Gematriya) has permission to prosecute 364 days a year - but not on Yom Kippur. But the Jewish calendar is based on the lunar year (approx. 354 days)?

What sins does the Satan not prosecute on Yom Kippur? Does the Yetzer ha'Ra incite us to sin on Yom Kippur - in the times of the Mikdash, and today?

In what event can the Satan prosecute even on Yom Kippur?

Do we rule halachically that Teshuvah is a prerequisite to achieving the atonement of Yom Kippur? Is there ever an instance in which Teshuvah is not required for atonement?

How does Rebbi derive that Yom Kippur itself atones, even without Teshuvah? What then is the benefit in doing Teshuvah?

What are the ramifications of repeated transgression? Is atonement still possible?

In what way is Teshuvah comparable to purification in a Mikveh?

How is Yom Kippur the culmination of the 10 days of Teshuvah? How does the Kohen Gadol achieve Kaparah for all of Yisrael? What does his entering the Kodesh ha'Kodashim represent? Why must no man enter the Kodesh during his Avodah? How does Yom Kippur atone even for those who do not repent (according to Rebbi)?

What do each of the Yom Kippur offerings atone for, and for whom?

In the absence of the Avodah nowadays, what atones for us on Yom Kippur? What is a prerequisite?

Do the unique Korbanos of Yom Kippur atone independently, or in conjunction with the day itself? What sin do most of these Korbanos atone for, and why are so many of these Korbanos necessary?

Why is the custom to omit "Mizmor l'Todah" on Erev Yom Kippur? How is this connected to the topic of which sins the Yom Kippur offerings atone for?