Why does the Torah present the Shelamim of a lamb and of a goat separately?
Rashi: Because whereas the fat-tail of a lamb is brought on the Mizbe'ach, that of a goat is not.
Why does the Torah sometimes call a lamb Kesev, and sometimes Keves?
Refer to Bereishis 39:40:151:1 and the note there.
Rashi writes that the Torah teaches a lamb and a goat separately, because they differ about the tail. It should teach a goat with cattle. There is no difference between them!
Moshav Zekenim: Cattle are not like goats at all. However, sheep and goats were taught together regarding Olah. Also here they would be taught together if they did not differ about the tail.
Why does it say "Im Kesev Hu Makriv"?
Moshav Zekenim: The Sifra expounds Im, and the the prefix Vov. This is unlike Rashi, who says that "Kesev" is extra, for Tzon is sheep and goats, and the next Pesach discusses goats. (In our texts, there is no prefix Vov, and Sifra expounds Kesev and Im Kesev, like Rashi