Dear Rabbi Kornfeld,
On daf 4b the gemara establishes that there is a hekesh that is makish all korbonos to shelamim - zos hatora, both for shinui kodesh and baalim, including chatas and asham. Why then, on 5b, does the gemara ask, mai chazis d'akshes (asham) lishlamim (and it will be kasher aval lo alu l'baalim..), akish l'chatas (and it will be totally pasul), and then gives the answer of the miut. That halacha was already included in the hekesh. The hekesh was makish ALL korbonos, including both asham and chatas. If the gemara was in the mood to ask anything at all, why not ask "akish CHATAS lishlamim (with the effect, of course, that the chatas, at least, will be kasher), and then give the answer of the miut "osah".
The Gemara on 8a actually uses the Hekesh to teach us that Chatas is Pasul; since the verse teaches she'Lo Lishmah by Chatas itself, and, in addition, it is part of the Hekesh, it comes out that the Din has been repeated ("Shinah Alav ha'Kasuv l'Akev"). The Hekesh is for the requirement of Lishmah; the matter of whether it is Kasher or Pasul is an independent issue.