Why did Moshe call Dasan and Aviram?
Rashi: He called them to him in order to appease them. 1
Ramban: He called them to warn them and their cohorts and to appease them. 2
Targum Yonasan and Rashbam: He sent messengers to invite them to a Din Torah.
Why did Moshe make a point of sending a Shali'ach?
Mo'ed Katan, 16a: With reference to 16:12:1:3 - Because it is the Shali'ach Beis-Din who must invite a litigant to a Din Torah and not the other litigant. 1
See also Torah Temimah, note 8. See also Torah Temimah note, citing the same Gemara, which learns from the Pasuk there the basic obligation of inviting the litigants to a Din Torah - in the name of a great man (See Torah Temimah, note 13), and of naming the other litigant and the exact time when to appear in Beis-Din - seeing as there, Moshe, who was calling Korach and his men to a Din Torah with Hashem, was the Shali'ach Beis-Din - Oznayim la'Torah, citing the Ran in the name of the Ra'avad.
Why did Moshe call Dasan and Aviram, but not On ben Peles?
Ramban: He called Dasan and Aviram because, together with Korach, they were the main instigators of the rebellion ? princes of the congregation and highly influential - whereas On simply followed them, and Moshe figured that if they would relent, so would he. 1
What are the connotations of "Lo Na'aleh"?
Rashi: Dasan and Aviram predicted that they would go down to below the ground. 1
Oznayim la'Torah: It seems from here that even in the desert it was customary for a leader and a Dayan to sit on a raised locaton. 4
In keeping with the saying 'Al Yiftach Adam Piv le'Satan' (B'rachos, 15a).
Rashbam: As we find in Ki Seitzei Devarim, 25:7 and in Shoftim, 4:5.
See Oznayim la'Torah, who explains the Pasuk in Ki Seitzei (See previous note) in the same way, and who interprets the Pasuk in Bo Sh'mos, 11:8 - in connection with Par'oh - "ve'Yardu Chol Avadecha Eileh Eilai" in similar fashion.