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: Moshe sent messengers to invite them to a Din Torah.
Why did Moshe make a point of sending a Shali'ach?
See 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 not call On ben Peles?
Ramban: After the initial stage of rebellion, On went home. Moshe did call him because he was subordinate to Dasan and Aviram, and he figured that if they would relent, so would he. 1
What are the connotations of "Lo Na'aleh"?
Rashi: By implication, they 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).
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.