Why did Reuven refer to "this pit that is in the desert"?
Ramban, Malbim: The pit was very deep, so Yosef would not be able to escape 1 and it was in a location where nobody was likely to hear his screams. Ha'Emek Davar - also, it was far from where the brothers were staying, so Reuven would be able to return Yosef to Yakov without their knowledge.
Hadar Zekenim: He wanted them to cast Yosef into a pit that did not contain snakes and scorpions. However, they ignored his request. 2
Oznayim la'Torah: The brothers said to throw Yosef into one of the local pits, because it made no difference to them into what sort of pit they threw him. 3 Reuven however, who intended to come back and save him, pointed to a specific pit which (a) did not contain snakes and scorpions, and (b) which he would later be able to identify.
Moshav Zekenim rejects Rashi who says in Bava Kama 50b, that a Stam Bor in the Torah is 10 Tefachim deep, because if so, Yosef could have easily escaped!
Riva concludes unlike this, for "ha'Borah" connotes the pit discussed until now.
Moreover, the words "Lechu v'Nahargeihu, v'Nashlichah b'Achad ha'Boros" implies that they should kill him and throw his corpse into one of the local pits.
What prompted Reuven to save Yosef's life and to even attempt to return him to his father?
Rashi: His sense of responsibility in his capacity as the Bechor, 1 because he realized that the blame would fall on him.
Bereishis Rabah 84:15: Reuven thought that he was expelled from Klal Yisrael due to the episode with Bilhah. However, when Yosef related his dream and counted him with the brothers, he reassured him that this was not the case. And it is due to Hakaras ha'Tov that he tried to save him. 2
Oznayim la'Torah: Considering that the brothers' hatred was based on the fact that Yosef wanted to rule over them - by taking the kingship and the Bechorah, one would have expected Reuven, who, at that stage, was heir to the throne and the Bechorah, to be the first to call for his death. The fact that it was specifically Reuven who saved him demonstrates an incredible display of good Midos. 3
Ha'Emek Davar: We should not punish him for his words and dreams - this is mere childishness! This is why he said "ha'Yeled Einenu", "Al Techete'u va'Yeled" (37:30, 42:22). However, they did not accept this.
Even though he seems to have somehow known that he would lose the birthright to Yosef. Refer to 29:32:1:1.
Mishnas R. Aharon 3 p.111-2: Reuven was the Bechor. He should have been the most upset that Yosef tells his dream! Rather, his spirit was very low due to Teshuvah, therefore he understood unlike his brothers. Hearing that they will bow to Yosef caused Reuven to love him, and caused the others to envy him.
How did Reuven convince his brothers to fall in line with his suggestion?
Ramban: By suggesting that they do not kill him directly. He said "Dam", and not Damo, to teach that they should not kill anyone directly; [and implied that] he does not say so amidst compassion for Yosef. 1
Refer to 37:21:1:3.
Seforno: He said that they should not kill him in a cruel manner (rather, let him die automatically 2 in the pit).
Malbim: One must be more careful about something explicit in the Torah - "Shofech Dam ha'Adam" (9:6). 3
Ramban: As a matter of fact, initially, it was only after they rejected his initial proposal to leave him alone altogether, that he suggested throwing him into the pit. See Bereishis 42:22.
Gur Aryeh: Moreover, by killing him indirectly, they would reduce the severity of the act from a one that carries with it the death sentence b'Dinei Adam to one that is only b'Dinei Shamayim.
Why does the Torah make a point of mentioning "Lema'an Hatzil oso mi'Yadam... "?
Teshuvas ha'Rashba Siman 981: To teach us that one should record and publicize someone who performs a Mitzvah.