The kids were given a gift. That they would go to find mother together. As far as I know when Kadaj found Rufus had Jenovas head they snapped completely. As for Sephiroth coming back with just Kadaj it's simply because Jenova had the ability to turn into something else. S/he knew Sephiroth was the best and called him. Aerith rain was probably on an as needed bases. She didn't know if she healed the stigma if it would really save the world. The remnants might just recall a virus or do something else to end the world. Right when she did she put her faith that cloud would do what he ended up doing... saving the world.

My theory is not proof. It's just my theory.