Who were the "Moshlim"?
Rashi and Ramban #2 (on Pasuk 29, citing the Midrash Rabah): "The Moshlim" (the parable-tellers) refers to Bil'am and his father Be'or. 1
Ramban #1: Refer to 21:14:2:2*.
Seforno: They are people who interpreted the parables that they dreamt, such as Bil'am. 2
Rashbam: It refers to people who predicted the future, such as Bil'am and his contemporaries.
Targum Yonasan: It refers to Tzadikim, who rule over their Yeitzer ha'Ra. 3
To whom did they say 'Come to Cheshbon ... !'
Seforno: The Moshlim are telling the inhabitants of Ar, the capital of Mo'av, to come to Cheshbon, the capital of Sichon and to submit to his sovereignty. 3
Targum Yonasan: Refer to 21:27:1:5. They said to themselves 'Come let us make a Cheshbon 4 - the gains of an Aveirah against the punishment, and the losses of a Mitzvah against the reward. The one who awakens and speaks words of Torah will be established!'
According to the Ramban, the Moshlim predicted that Sichon would come and destroy Cheshbon and subsequently rebuild as his own capital.
Rashi: When Sichon hired him after failing to defeat it. And it was as a result of that curse that Balak said to Bil'am (in Bamidbar, 22:6) "Because I know that whoever you bless is blessed and that whoever you curse is cursed.
From the fact that the Rashbam refers to Ar as the capital of Mo'av, he seems to explain the Pasuk like the Seforno.
See also Ba'al ha'Turim.