Why did Reuven refer to "this pit that is in the desert"?
Ramban: 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.
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 identfy.
Moshav Zekenim rejects Rashi who says in Bava Kama 50b, that a S'tam Bor in the Torah is 10 Tefachim deep, because if it is, Yosef could have easily escaped!
Riva concludes unlike this, for "ha'Borah" connotes the pit discussed until now.
Moreover, the Lashon "L'chu Venahargeihu, ve'Nashlichah le'Aches 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 B'chor, 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
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 B'chor. 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?
Moreover, by killing him indirectly, they would reduce the severity of the act from a one that carries with it the death sentence be'Dinei Adam to one that is only be'Dinei Shamayim. See Or ha'Chayim.
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 sand publicise someone who performs a Mitzvah.
רש"י: רוח הקודש מעידה: ממה בא רש"י להוציא?
גור אריה: אפשר לטעות שכוונת ראובן הייתה שיוסף ימות בבור והם לא ישפכו דם בידיהם, ומה שכתוב "למען הציל אותו" פרושו שה' עשה שכך יאמר ראובן כדי להציל את יוסף. כדי להוציא מהבנה זו פירש רש"י שרוח הקודש מעידה שראובן אכן רצה להציל אותו.
רש"י: להציל אותו: בבור היו נחשים ועקרבים, וכתוב בגמ' שהנופל לבור מלא נחשים ועקרבים אשתו מותרת להנשא, א"כ כיצד חשב ראובן להצילו?
גור אריה: היו בבור נחשים ועקרבים אבל הוא לא היה מלא מהם ואפשר שיוסף יצליח להמית אותם.
כיצד שיכנע ראובן את האחים בטענת "אל תשפכו דם" והרי הוא עתיד למות שם מרעב?
גור אריה: ראובן אמר להם שרק יגרמו לו למות ולא ימיתו אותו בידים.