Why did Hashem find it necessary to instruct Yisrael to bring the Korban Pesach? They offered in the Midbar even obligations without a fixed time!
Rashi: Because Hashem initially told them 1 them that they would bring the Korban Pesach when they entered Eretz Yisrael.
Rashbam: Because Pesach Mitzrayim lasted only one day and the Korban Pesach was different than it was originally intended to be brought. 2 It was therefore necessary to tell them to bring it in the regular manner.
Seforno: In spite of the Simchah of the eighth day of the Milu'im and the Chanukas ha'Mizbe'ach, they were to bring the Korban Pesach. 3
Having told them that they would bring the Korban Pesach after they entered Eretz Yisrael, why did He command them to bring it now?
Ramban #1: Because He wanted them to have a remembrance of the redemption and the miracles that they and their fathers had just experienced, to hand down to their children, and their children to their children. 1
Why did Yisrael not bring the Korban Pesach during the 40 years that they spent in the desert?
Rashi: Refer to 9:2:1:1.
Ramban #1: Refer to 9:1:2:1**.
Ramban #2: Simply because they were lax and did not take the trouble to bring it. 2
Ramban: As a result of which they were placed in Cherem and the north wind did not blow. Consequently, they were forbidden to eat Kodshim. It is obvious however, that they kept the Yom-Tov of Pesach and did not eat Chametz for seven days - seeing as it is a Chovas ha'Guf (not connected with Eretz Yisrael) and applies everywhere.
The Ramban himself however, prefers the previous answer, inasmuch as Moshe would never have allowed Yisrael to transgress a Chiyuv Kareis and would made sure that Yisrael would bring the Pesach had it been permitted.
How were Aharon and his sons able to offer the Pesachim of all Yisrael, and Chagigos in order to eat Pesach when satiated?
Moshav Zekenim citing R. Yehudah of Shapira: According to the opinion that Pinchas became a Kohen only after he killed Zimri, we can say that Aharon had many other grandchildren. This is unlike the opinion that Se'ir Rosh Chodesh was burned because it could not be eaten due to Aninus. 1
Moshav Zekenim: Yisrael did Shechitah, and Moshe and Aharon and his sons did Kabalah and Zerikah. However, we cannot say that Moshe was a Kohen according to the opinion that Se'ir Rosh Chodesh was burned due to Aninus.
Moshav Zekenim citing Ri: Yisrael had giant lambs that they offered for the Tamid. When they carried them on poles, the legs dragged on the ground. If so, they could slaughter a small amount (e.g. 1000), and everyone would get a k'Zayis. 2
Many verses about making Bigdei Kehunah and the Milu'im say "Aharon u'Vanav." We can say that grandsons are included. However, sometimes it names them, e.g. "Lechahano Li Aharon Nadav