Ki Tavo 1 - The shevatim of Har Gerizim and Har Eival

Why are the 12 shevatim divided as they are at Har Gerizim and Har Eival for the blessings and the curses described in this week's parsha - i.e., Shim'on, Levi, Yehudah, Yissachar, Yosef, and Binyamin on Har Gerizim, the mountain of the blessing, and Reuven, Gad, Asher, Zevulun, Dan, and Naftali on Har Eival, the mountain of the curse?

A commonality between the tribes of Yehudah, Yissachar, Yosef, and Binyamin is that all were ancestors of kings: Levi to the Chashmona'i kings (or, if one would rather, to the kohanim and levi'im, also significant personages) Yehudah to David, Yissachar to Ba'sha, Yosef to Yehoshua and to Yerav'am (and, according to P'sikta Rabbati 3, Yeihu), and Binyamin to Sha'ul. Shim'on may have merited inclusion by virtue of his close connection to Yehudah. I do not know of any sources as to the ancestry of the kings Zimri, Omri (although his choice of Shomron as a capital may imply that he, too, was from Yosef), Shalum, Menachem, or Pekach.

As another question, why are the tribes of Har Eival ordered as they are, with Zevulun in between the b'nei Zilpah and the b'nei Bilhah? On both mountains, the sons of Leah and Zilpah comprise the first four tribes of each set, and are in age order, while the last two tribes of each set are from Rachel/Bilhah - but I'm not sure what this might mean.