Why is the Torah lenient on the sinner here, allowing him to bring a smaller Korban if he cannot afford a bigger one?
Ramban: In the two cases of Shevu'ah, it is because they are not subject to Kares; Whereas in that of Tum'as Mikdash ve'Kodashav, it is due to the fact that he erred with regard to a Mitzvah (either whilst eating Kodshim or whilst going to the Beis-ha'Mikdash to bow down to Hashem or to bring a Korban). 1
Da'as Zekenim, Hadar Zekenim (4:3), Moshav Zekenim: He did not profit from his Aveirah, e.g. entering the Mikdash b'Tum'ah or a false Shevu'as Bituy 2
Moshav Zekenim: This is difficult, for if one had a baby to circumcise after Shabbos, and circumcised him on Shabbos, all agree that he is liable (Chatas Behemah, even if he is poor), since he had no Mitzvah to do [that overrides Shabbos
Why does the Torah require him to bring both Olas ha'Of and Chatas ha'Of?
Moshav Zekenim citing the Rosh, Ramban (citing Ibn Ezra): They are in place of Chatas Behemah, in which part goes [on the fire on the Mizbe'ach] to Hashem, and part goes to Kohanim. Nothing from Chatas ha'Of goes to Hashem, and Kohanim do not get anything from Olas ha'Of,