Where was Reuven when the brothers sold Yosef?
Rashi #1: It was Reuven's turn to serve his father that day.
Rashi #2 and Targum Yonasan: He was busy with his sack-cloth and with fasting (doing Teshuvah) 1 for having switched Yaakov's bed. 2
Malbim: Reuven was with the other brothers, away from Yosef. He intended to go to save him before the Yishma'elim arrived; but Hashem sent Midyanim to take Yosef before Reuven got there.
Targum Yonasan: In fact, he hoped that, if he returned Yosef to his father, he would forgive him for his sin.
Oznayim la'Torah (citing Pirkei d'R. Eliezer): Reuven went to meditate on one of the local mountains, with the intention of returning in the night to extricate Yosef from the pit and return him to his father. See Oznayim la'Torah DH 'va'Yashav Reuven' #3.
Why did Reuven rend his clothes?
Oznayim la'Torah: Because he thought that his brothers had killed Yosef unjustly. 1
Ha'amek Davar: Because he thought that Yosef had been killed (not via the brothers).
Oznayim la'Torah: Otherwise, he would not have mourned, since one does not mourn the death of people that Beis Din executes.
