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 (citing Sifra): Im Kesev includes the tail of a lamb of Korban Pesach. V'Im Kesev 1 includes a Pesach whose year passed, or Shelamim brought due to Pesach 2 for all Mitzvos of Shelamim - Semichah, Nechasim and Tenufah of Chazah v'Shok. However, it is eaten only for one day and a night, like the law of what it was initially Hukdash for. Ben Azai says, it is eaten only at night, and roasted, like Pesach.
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 - PF.)
Moshav Zekenim: This is Mosar Pesach, i.e. Temuras Pesach, or it was lost and the owner offered another in place of it. It is not the Chagigah of Erev Pesach (to be satiated from eating it before eating Pesach. Rashi (Pesachim 96b) brings both Perushim.)