Why did the brothers decide to throw Yosef into a pit?
Oznayim la'Torah: 'Midah k'Neged Midah' - Yosef wanted to rise above them, so they would throw him into a deep pit.
Considering the agony that that would inevitably cause Yaakov, why did they decide to tell him that a wild beast had devoured Yosef?
Seforno: In order to prevent him from cursing them.
Why did Shimon and Levi say "v'Amarnu" - in the past tense modified by a 'Vav ha'Hipuch' - rather than 'v'Nomar' - in the future (like the other terms in this Pasuk -- "v'Nahargehu," "v'Nashlichehu" and "v'Nir'eh")?
Oznayim la'Torah: Granted, they decided that Yosef was Chayav Misah for his having spoken Lashon ha'Ra about them. However, they dreaded the deep anguish that they would cause father by informing him that his beloved son was dead. Therefore they said it in the past, as if to say 'We wish we had already informed him... !' 1
Refer to Bereishis 23:13:2:1 - where Rashi presents a similar idea.
Now that the brothers planned to kill Yosef, how could they say, "v'Nir'eh Mah Yih'yu Chalomosav"?
Rashi (citing Bereishis Rabah 84): It was not the brothers who made that statement, but Hashem. They said "Let's kill him;" and Ru'ach ha'Kodesh retorted, 'Let Us see whose word will be fulfilled!'
Ramban #1, Seforno, Targum Onkelos and Targum Yonasan: They said sarcastically, 'Let's see whether, after his death, we will bow down to him!'
Ramban #2: 'Let's see what happens to his dreams,' they were saying; 'If he is saved from our hands, then we will indeed bow down to him.'
Ohr ha'Chayim: This will prove that his dreams were fabricated, or due to his thoughts during the day (and were not a prophecy).
Malbim: If his dreams were true [prophecies], Hashem will save him.
Refer to 37:20:154:1.
Why does it say "v'Atah Lechu"?
Ohr ha'Chayim #1: Let is go towards him [to kill him quickly]. They could not bear to wait until he reaches them.
Ohr ha'Chayim #2): "V'Atah" hints that in the end, they will repent. It refers to Teshuvah (Bereishis Rabah 21:6).
Why does it say "v'Nahargehu"?
Ohr ha'Chayim: We will kill him together. If 10 killed someone together, all are exempt in Beis Din (Bava Kama 26a). We are [also] exempt bi'Yedei Shamayim, for he is liable to die for being an Ed Zomem (refer to 37:18:151:7). They needed an exemption from Beis Din, for 'Ein Lahem Hazamah.' 1
Ohr Yakar: I.e. Hazamah does not apply to Bnei Noach. (I.e. the Ed is liable bi'Yedei Shamayim (refer to 37:18:151:7*), but Beis Din does not kill him, so the brothers would be Chayav Misah bi'Yedei Adam. Alternatively, they did not have Mezimim, so they could not prove that Yosef was an Ed Zomem. However, why did they fear lest Beis Din kill them? Did they plan to kill Yosef in front of witnesses?! - PF)
Why will they say that an evil Chayah ate him, and not that it killed him?
Ohr ha'Chayim: They did not want to lie. Presumably, a Chayah such as a weasel would eat him in the pit [after they killed him].
Rashi (citing a Midrash) writes that the brothers did not say 'let us see...', rather, Hashem. Why would Hashem need to say so? The Torah says what happened!
Ha'amek Davar: The verse does not leave its simple meaning. Hashem put these words in their mouths. Hashem caused that they not pay attention to their own words. This teaches that one should pay attention to words that he says without knowledge or intent!
