Which Korbanos is the Pasuk referring to?
Rashi and Ramban: it is referring to the Musafin that are recorded in Pinchas.
Why does the Pasuk allude to them here?
Rashi and Ramban: To teach us that the various parts of the Korban are not dependent upon one another - If there are no bulls, bring the rams; if there are no rams, bring the lambs. 1
Seforno: To incorporate Chol ha'Mo'ed in the realm of the Musafin.
Ramban and Seforno: It does not list them here because it lists them in Pinchas.
Why does the Torah write "Shiv'as" and not 'Shiv'ah'?
Rashi: 'Shiv'ah' is an adjective; "Shivah", a noun - a group of seven. 1
See also Sifsei Chachamim.
What is the definition of "Meleches Avodah"?
Ramban #1 (on Pasuk 7, citing R. Chananel) and : All Melachos that are not for Ochel Nefesh, 3 and that are for the purpose of acquiring, such as sowing and reaping, digging etc.
Rashbam: A Melachah that entails a lot of bother.
Rashi: Davar ha'Aveid (which is permitted on Chol ha'Mo'ed - Sifsei Chachamim [See also Ramban]).
See Ramban's objections to this explanation, one of which, based on Chagigah, 18a, is that the Melachos that are forbidden on Chol ha'Moed (Refer to 23:8:1:1*) are not hinted in the Torah but were handed down to the Chachamim to decide what is forbidden and what is permitted.
See Ramban, who elaborates.
Why does the Torah group Shabbos, Pesach and Chag ha'Matzos in one Dibur, whereas each of the other Yamim-Tovim is presented separately?