Why does it say "Im Ya'aleh"? They are commanded to ascend!
Radak: It means when they ascend. Also "Im Yihyeh ha'Yovel" (Bamidbar 36:4) means when there will be Yovel.
Why did Yaravam fear lest people return to Rechav'am when they go to Yerushalayim for the festival?
Rashi (from Sanhedrin 101b): (a) Only kings from Beis David may sit in the Azarah. He will sit, and I will stand. (b) In Motza'ei Shevi'is, the king reads Parshas ha'Melech 1 (Hakhel - Devarim 31:10-11). He will read it, for he leads the locale 2 . I will be like the rest of the nation (commoners)! I will be disgraced.
Radak: When they ascend, he will veer their hearts to return to him, for he is their master, a Melech ben Melech.
Radak (from Yerushalmi Avodah Zarah 1:1): Yaravam became king in Motza'ei Shevi'is. (This is less than two weeks before Sukos. It takes some people two weeks to return from Yerushalayim to their homes (Ta'anis 10a. Presumably, they already starting heading to the Mikdash! Perhaps he immediately sent Sheluchim to inform people not to ascend to Yerushalayim. They found on the road some of the people who already started ascending, and missed some, and told the others in their homes. - PF)
Radak: The king of the city has precedence. If I will read second, it is dishonorable for me; if I will not read, it is a disgrace for me!
Why did he fear lest they kill him?
Radak: They will think that I am not a king (refer to 13:27:2:1 and the notes there), and kill me for Mored b'Malchus.
