1)
Why does it say "u'Mehem Min Bnei Shimon"?
1.
Malbim: Even though the leaders were from Menasheh (refer to 4:42:2:1), Bnei Menasheh did not join in inheriting there.
2)
Who are these Bnei Yish'i?
1.
Radak, from Bava Basra 123b: They were from Menasheh. The seed of Esav will fall only to the seed of Yosef, for it says "u'Veis Yosef Lehavah." How did Bnei Shimon defeat Esav? Bnei Yish'i were from Menasheh - "v'Eleh Rosh Beis Avosam v'Efer v'Yish'i?" (5:24).
