The first question can be answered on the basis of Rashi’s comment derived from earlier sources (Sanhedrin 110a), that originally they joined in the revolt but then withdrew before it was too late.
Why this is important is that it explains why and how they are able to figure in later history when they were “inspired singers of the nation”, as Samson Raphael Hirsch describes them – they are listed as authors of twelve chapters of the Book of Psalms – and Temple officials (the Books of Chronicles name them as musicians, gatekeepers and even bakers).
Korach’s descendants included Samuel, which shows that even villains can have good children (cf. Talmud Gittin 57b). The sages said that Haman’s descendants taught Torah in B’nei B’rak, Sisera’s taught children in Jerusalem and Sennacherib’s included scholars such as Sh’mayah and Avtalion.
People can rise above their ancestry. They can say, “I can throw off the baggage I came with and make my own positive contribution”. Of course it can happen the other way too; one can be weighed down by the baggage and never escape the jinx. The rabbis say, “Your own deeds will bring you closer to people: your own deeds will distance you from them” (Eduyot 5:7).
The Mishnah warns people not to remind anyone that their ancestors were wicked people (Bava M’tzi’a 4:10). Everyone must be given a chance to make their own way in society.