I think I got enough solidity to the core shape to do what I need.
The cast is in soft urethane, which is extremely pliable. And unbelievably sticky. Jokes aside, it's a difficult material to work with-- it's almost a gel.
Destroyed the mold even further. The original cracking turns out to be a fatal flaw.
That last one isn't as bad as it seems, if I can work with the urethane casting. I'm going to try and clean up and repair the mold one last time and pull one last "stunt torso" out of it with yet a different set of materials (latex and foam urethane). That doesn't need the detail as much, so the flaws that will show up should be okay.
It does however ruin my nefarious scheme of selling the SybilReplicants on ebay. Sorry thegooderic.
(Actually, the mold was always planned to be destroyed, just not in the process of casting from it.)
1.) Direct plaster does not work very well with a torso cast of a live model. Either kill the model beforehand, use a different material, or add something to the plaster to make it pliable and less likely to crack.
2.) Urethane which sticks to everything else apparently doesn't stick to cured latex very well.
3.) Latex contracts as it dries. This has the unfortunate side effect of flattening concave surfaces, and when the concave surfaces are important, it's a bad thing.
4.) My hand spread open wide can span the narrowest point of sybildiscontent's waist. While I do have big-ass hands, that is just freaky weird. It's like one of those optical illusions where the two things look completely different but they're actually the same size.
I am back now to bed going.