If the sons of Korach joined their father in the plot against Moshe, why did they not die?
Rashi (citing Sanhedrin, 110b): Initially, the sons of Korach (Asir, Elkanah and Avi'asaf) were part of the plot, 1 only when it came to the crunch they entertained thoughts of Teshuvah. 2
Targum Yonasan: 'The sons of Korach did not participate in their father's rebellion (because) they followed the teachings of Moshe the Navi. Therefore, they were neither burned by the fire nor were they swallowed up by the earth'.
Rosh (16:25): They were not with Korach. They were Talmidim of Moshe, and they knew that all that he does is from Hashem.
See Sifsei Chachamim.
Rashi (Ibid.): "Therefore an elevated place was reserved for them in Gehinom and that is where they sat. Rashi (in Tehilim, 42:1): 'Asir, Avi'asaf and Elkanah (the sons of Korach) were initially part of their father's plot, but once the Machlokes took off, they separated from it ... their location remained intact inside the mouth of the earth, where they said Shirah, and that is where they established the chapters of Tehilim 42-49. Eventually, they emerged from there, Ru'ach ha'Kodesh rested upon them and they prophesied about the Galus, the Churban Beis-Hamikdash and Malchus Beis David'. See also Torah Temimah, note 4
Seeing as the sons of Korach did Teshuvah, why did they have to dwell alone in a reserved place in Gehinom?
Bartenura: Midah ke'Neged Midah, because they joined in the feud and caused others to separate from one another. 1
This is akin to Chilul Hashem, for which Teshuvah, Yom Kipur and afflictions do not atone (Yoma 86b), since they caused others to sin.- PF.
Did the sons of Korach receive a portion in Eretz Yisrael?
