Hey, I know you can become multiple types of necromancer, but if you do, can you turn someone into multiple types of zombie, thus making it stronger each time?
hmmmmmmm
If you mean as sentient undead, it's very hard to kill one and leave a body that can be raised as a sentient undead a second time. If you mean as an ordinary zombie, I think this requires some science. I'm not sure whether animation interactions get cleared when the creature dies. I suspect they don't, and you can kill a zombie hand or something six times and raise it different ways until it becomes unstoppable.
Ice. I did a lot of thinking to figure out the best way to kill and raise my adventurer multiple times, and what I settled on is that getting encased in ice kills you no matter what, yet doesn't damage the body. It's kind of a pain in the butt, but it works. You do have to pick up the body and take it somewhere else first though. For some reason, the game doesn't like reanimating a freshly-thawed corpse.
Based on my own experience, you definitely
can become multiple types of sentient undead simultaneously. You retain all your previous powers when resurrected a second time. I don't know how to check the numbers to confirm this, but based on the fact that after the second reanimation, my heavy kicks were able to send goblins and elves flying several tiles (punches too, albeit far less consistently), I suspect the strength bonuses stack.
Unfortunately, it might be a bit buggy. My adventurer just sort of vanished while I was using tactics mode. The tile where he was standing still causes people to lie down when they enter it, but I can't switch to control him or interact in any way. The game still thinks I have a 2-character party, but the first slot is empty.
As for reanimating NPCs, decapitated corpses can be killed by a stiff breeze. So if you're sticking to dumb undead, that's pretty easy. Granted, it may not be worth it if you want something tough. It gets trickier with intelligent undead. They can't talk, so getting them to follow you requires some finagling. And you have to make sure they're not hostile to you when they die, or they'll remain hostile upon reanimation. And, again, since they can't talk, you can't ask for a cease to hostilities.