What is "Cheilek ke'Cheilek Yocheilu" referring to? What is it coming to teach us?
Rashi: Refer to 18:6:3:2. It teaches us that Kohanim who come to the Beis-Hamikdash during Yom-Tov, even when it is not their turn to serve, share in the skins of the Korbanos of Yom-Tov and the meat of the Se'irei Chata'os of Yom-Tov. 1
Seforno: It means that the Mishmaros (groups) of Kohanim, each of which served one or two weeks in the year in the Beis-Hamikdash, would only receive a portion in the Korbanos of the year what they traded with each other 2 ('We will receive your portion this week, when we are serving, and the week that you serve, you will receive our portion'). 3
Toras Moshe: It implies that a Ba'al Teshuvah receives a share in Gan Eden like Tzadikim. Refer to 18:6:151:3.
Sukah, 26a: With reference to the Lechem ha'Panim, teaches us that when Yom-Tov falls on Shabbos or on Shabbos in the middle of Yom-Tov, all twenty-four groups of Kohanim share in the Avodah and in distribution of the Lechem ha'Panim. 4
What is "L'vad Mimkarav al ha'Avos" coming to teach us?
Rashi: It precludes the Tamid and the Musaf of Shabbos during the Regel and Nedarim and Nedavos, which the Kohanim whose turn it is that week sacrifice. 1
Seforno: Refer to 18:8:1:2. It teaches us that during the year, when it is possible to exchange one week for another, the Avos instituted Mishmaros, where one group swaps with another. 2
Rashbam: It precludes a Kohen, who is permitted to bring a Korban even when it is not his turn to serve in the Beis-Hamikdash
Rashi: As per the agreement that their Avos made in the time of David and Shmuel, when the Mishmaros of Kohanim were instituted, whereby the groups sold the rights to one another 'You take your week and I will take my week!'
The Seforno does not explain who the Avos are. On the other hand, see note on answer #1, and Targum Yonasan, who names the Avos as Elazar and Isamar - who initiated the Mishmaros. See Oznayim la'Torah, who elaborates and who points out how the Torah hints at the Mishmaros which Elazar and Isamar instituted hundreds of years before it became necessary to put them into practice.
Rashbam: As in Melachim 2, 12:6.
See Na'ar Yonasan and K'li Yakar.
