Well I restarted just now and tried importing my custom world_default as an experiment -- and wouldn't you know, it actually worked again once.
I tried it again, though, after a second restart, and it crashed after getting the "would you like to import names?" phase.
However, importing a custom names.xml using the the "Re-import name and ethnicity database" function has yet to crash the game.
So between all of that and the fact that a stock world_default.xml has crashed it too, I'm still thinking it's something with the game itself.
Devs: I can send the crash log or any additional info if that helps. It shows "EXC_BAD_ACCESS (SIGSEGV)" and "KERN_INVALID_ADDRESS."