How did the rabbis marry up the teaching that all Israel has a portion in the world to come with the need for repentance and feasts such as Yom Kippur?

One thing to keep in mind is that a great deal of the teachings of Judaism is not based upon Scripture but on rabbinic interpretations of certain Scriptures. According to the Scriptures, there will be a time when all Israel will be saved. However, instead of limiting this biblical teaching to a specific future generation, the ancient rabbis applied it to all generations of Israel and taught that because all Israel is righteous, all Israel will enter into the Messianic kingdom. This teaching was handed down from one rabbi to the next over numerous generations, and it is the traditional teaching even today. As for Yom Kippur, the rabbinic teaching is that this holy season determines if a Jew will live or die during the following year. Yet, regardless of any disciplines in this life, they teach that all Israel will be righteous upon the resurrection of all Israel, and therefore all Israel has a share in the age to come.