How do we reconcile this Pasuk with the Pasuk in Re'eh "Sheishes Yamim Matzos Tocheilu"?
Rashi, based on the Mechilta: From the latter Pasuk we learn that there is no obligation to eat Matzah on the seventh day of Pesach. 1 And since the current Pasuk writes seven days - comparing the six days to the seventh, it teaches us that the non-obligation extends to all seven days. 2 This leaves us with the obligation to eat Matzos only on the first night, which we learn from Pasuk 18 "ba'Erev Tochlu Matzos". 3
Though the Isur of eating Chametz remains (Rashi).
This is based on the principle 'Kol Davar she'Hayah bi'Chelal, ve'Yatza min ha'Kelal Lelamed, Lo Lelamed Al Atzmo Yatza, Ela Lelamed Al ha'Kelal Kulo Yatza' (Rashi [See Sifsei Chachamim]).
Which is otherwise superfluous (See Sifsei Chachamim). See also Ba'al ha'Turim (on Pasuk 18).
What exactly, does "Tashbisu" entail?
Targum Onkelos and Targum Yonasan: "Tashbisu" means to nullify (to declare null and void), not to destroy.
What does the Torah mean when it writes "ba'Yom ha'Rishon"?
Da'as Zekenim: On the first day (Yom Tov), you already eradicated Se'or [the day before]. This is like "va'Ychal Elokim ba'Yom ha'Shevi'i."
Why does the Torah add the words "ha'Nefesh Ha'hi"?
Rashi: To teach us that one is only Chayav Kares if one eats Chametz b'Meizid, but not unintentionally. 1
See Sifsei Chachamim.
Bearing in mind the Pasuk in Vayikra (22:3) which indicates that Kares means cut off from before Hashem (i.e. wherever Hashem is), why, here as well as by every other Kares in the Torah, does the Torah insert the word "mi'Yisrael" or "me'Adas Yisrael" or "Mitoch ha'Kahal" or "me'Ameha" or "mi'Kerev Amah" - implying that the Soul that sinned can go and join another nation?
Sifsei Kohen on the Torah, Bamidbar 19:13: The source of the souls of the Jewish People is the Creator, and thus the entire collection of Jewish Souls is actually One, just as the Creator is One. Any soul cut off from the Jewish People is thus cut off from the Oneness of Hashem.
Malbim (here): That is why the Torah writes once (in Emor) 'mi'Lifnei Hashem'. The Torah means to clarify that Kares does not leave the option 'join' the destiny of a different nation; the person is cut off entirely.
Rashi writes that this is 'Davar she'Hayah bi'Chlal, v'Yatza..." Here, the seventh day was not taught!
Refer to 12:15:152:2,3.
Moshav Zekenim (18), citing R. Yom Tov: The seventh day is equated to the six. The Mechlita said Davar she'Hayah bi'Chlal, v'Yatza - this is imprecise. 1
Moshav Zekenim calls this 'Gimgum' (unclear).
Rashi writes that this is 'Davar she'Hayah bi'Chlal, v'Yatza..." Always, the Klal and Prat are similar. Here, the Klal is Isur and the Prat is Heter!
Hadar Zekenim citing Ri of Ivra: Since it says "ba'Erev Tochlu Matzos", this implies that afterwards it is not a Chovah. This is unlike Rashi.
Hadar Zekenim citing R. Yom Tov: Here, the Klal is not Isur, for it is not clear whether the seven days are Chovah or Reshus. The Prat reveals that it is Reshus.
Hadar Zekenim: Some 1 explain that "uva'Yom ha'Shevi'i Atzeres... Lo Sa'aseh Melachah" (Devarim 16:8) was bi'Chlal, v'Yatza, (Before this (verse 3), it says "Shiv'as Yamim Tochal Alav Matzos.") The Torah mentioned Yom ha'Shevi'i only for the Isur Melachah, but not for the Mitzvah to eat Matzah!
Riva citing R"A: Here the Prat (the seventh day) was not taught explicitly, but it is learned from the Klal, so it is like Isur.
Rashi writes that just like six days are Reshus, all seven days are Reshus, just the first night is Chovah. We should say that just like the first night is Chovah, all six days are Chovah! If there are two ways to learn, we learn the stringent way!
Moshav Zekenim citing R. Tam: If so, the Torah should have written only six for Chovah, and seven for Reshus. Why did it teach also "ba'Erev Tochlu Matzos"? We must say that it is to teach that also six days are Reshus. 1
Hadar Zekenim, Riva: It is more reasonable that seven days teaches about six days, for both of them say "Yamim", and not nights. Also, ba'Erev Tochlu Matzos cannot teach about all seven days, for since the Torah taught six and seven, you are forced to say that the seventh is Reshus!
Moshav Zekenim: This is difficult. Let the Torah write only "ba'Erev Tochlu Matzos", and seven days for Reshus! Riva - we would say that the other nights are Reshus, but all the days are Chovah. Or, I would not know which night is obligatory; Sheshes Yamim teaches that the first night is obligatory.