Well, we could make it so when you kill the Necromancer, instead of the Zombie losing all power, they just become...
Yeah, but that would still be troublesome. Undead would become waaay to strong like that. You just have to put as many soldiers between your necromancer and your enemy as possible and you will never die.
As for Greenskins, they should indeed get extra damage with melee weps. In the 2001 rulebook is a chapter about orcs. They have stupidity and a CC bonus. I hate stupidity though, although I like it when my army of Greenskins get mad at eachother and attack themselves.
Should orcs and undead work together? Would they get along?