I've wanted to do a story one myself, but with the newer versions of the game, I never got around to it. For succession games, I honestly don't think they'd work very well. A lot of their stuff is large, and to make things look good requires a unified vision and at least some planning, especially since it can take a long-ass time to dig out a good-looking dungeon or materialize the blocks to build a proper menacing evil fortress or mausoleum.
Warlocks are backstabbing, conniving little bastards who hate everything and everyone (and only become friends due to how the game makes friends happen.) In a succession game scenario, the only way you'd be able to make a logical transition of power is by killing each overseer, which is obviously bad for gameplay. Course that's just my opinion based on how I regard them. Maybe it'd work out perfectly fine with absolutely no issues brought on by people actually roleplaying to some extent based on in-game personalities.
1. It was changed at some point that you'll get regular migrant waves, because your warlocks were often so drastically few in number it wasn't worth it (or sometimes it was even impossible,) to use some of their end game stuff, because it consumed a warlock you had no means of replacing.
2. Ghouls subsist on meat and need hydration and rest. Zombies and skeletons didn't eat or drink anything, or sleep. Warlocks behaved like normal units, with no special food requirements.
3. Early attacks can come as soon as summer, actually. It's totally random. Earliest one I had hit me in
Hematite, comprised of a group of lightly armed humans.
4. Warlocks I don't recall looking identical, but eh. Just leaves room for creativity.
5. They can use souls harvested from fallen enemies to trade for powerful allies and high grade materials. For seige summoning, I doubt it'd be needed unless you have something specific you're doing and there's not enough souls stored in phylacteries (which they can use to keep souls "fresh," made from rock if I recall right.)