BTB - Lure of Fortune & Chaos Dwarfs, some other stuff

I'm planning on running a Border Town Burning campaign later this year, and my friends and I have been discussing potential warbands. I quite like the idea of playing a band of Chaos Dwarf slavers, so I'd be going the 'Lure of Fortune' and Engine of Chaos route. I was wondering, as I'd need to capture 12 slaves as one of my win conditions, whether this means I'd eventually need a second Engine of Chaos (as they only hold 6 slaves), or if slaves that are sent back to the Dark Lands count towards this total?

I'm probably looking at around 6-7 players (possibly more). So far there's me with the Chaos Dwarfs, a Merchant Warband and some (edited) Shadow Warriors all interested in the Lure of Fortune objective. We've also got a Scion of Chaos Nurgle-aligned Beastmen warband and a Scourge of the Realm horde of Restless Dead, and two undecided players, one of whom is leaning towards Carnival of Chaos. One question that has been asked is does the campaign need people playing Celestial Protectorate or one of the other 'good guy' plots to function properly, or will we be fine with what we have?

The 12 slaves definitely include any that have been sent back to the Dark Lands or sacrificed.

I think it would be really good to have a Celestial Protectorate warband but I'm sure you'll have fun without one. The Scourge of the Realm warband will help keep the Scion of Chaos warband(s) in check.